Q:

Why might the vast majority of my alien species be incredibly kind and compassionate?

For starters, I’ll describe what I’m going for with this species (I’ll call them White Hairs because, you guessed it, they have white hair) They are humanoid with pale gray skin, tall and pointed, but stationary ears, and are taller than humans on average by a few inches. White Hairs are a very tight knit species, but aren’t shy with humans (those in my ‘world’ live in a ‘new’ galaxy) or other species; in fact, they’re quite outgoing.
As the question’s title states, the vast majority of the White Hairs are incredibly kind and compassionate. To clarify, I’ll give an example: Let’s say that a human walks into another human’s home and shoots him. These two humans don’t know each other, but, for whatever reason, the killer decided to shoot this stranger. If you stopped a random White Hair on the streets and told them about it, their first thought would be to wonder what happened to the killer in their life to bring them to do such a thing (were they coerced into it by a gang of equally pitiable persons, has their life been so full of hardships that they are now filled with hate, have they taken an offer of money in exchange for killing this man because they badly need it, etc.) They will always attempt to understand why others do what they do and will be sympathetic towards them, no matter how heinous their crime. Now, let’s say that it’s been proven that the killer is a sociopath, that he lacks any and all remorse for what he’s done, that there is nothing in his reason for killing the victim that your average human could sympathize with (he’s had a blessed life, has plenty of money, and isn’t even associated with any criminal organizations) Even in this situation, a White Hair would not feel disgust towards the killer, they would pity him because he is incapable of empathy, which they hold in the highest regard, and love, which they view as something to be cherished and grateful for.
Now, by ‘the vast majority’, I mean such a high percentage (I’m thinking around 98% or more) that practically all humans and members of other species view the White Hairs as the bleeding hearts of the galaxy as a whole. They consider it a fact that literally all White Hairs are this way and you would be hard pressed to find someone that disagrees.
I first considered the explanation that they are simply raised this way, that it is so concrete in their culture that it is impossible to grow up in a White Hair household without having this mindset instilled into you. The more I thought about this, though, I began to see a flaw in that idea. If the White Hair way is simply the product of upbringing, wouldn’t there be plenty of White Hairs that didn’t genuinely have this disposition, but just acted as if they did or strived to make themselves think this way because it’s their definition of normal?
This brought me to the notion of there being a genetic reason for their mindset. I have a problem with a genetic explanation as well, however: I’ve come up with two White Hair characters that have unusual dispositions compared to the rest of their species. One of these White Hairs deviates greatly from the rest and has to act when around family and others because she feels that there’s something wrong with the fact that she’s unusually selfish and less compassionate than her peers. The other unusual White Hair is genuinely kind according to human standards, but if someone punched him in the face his initial reaction wouldn’t be to wonder whether or not that person got enough love as a child, he would just get mad, because that hurt, so screw you. I’m not sure how to explain the existence of this small percent if their disposition is a genetic thing or even if disposition can really be effected by genetics. 
I would really like to be able to explain this through upbringing and culture because I’m disinclined towards genetic engineering or eugenics as the cause. However, I can’t figure out how to avoid the likelihood of many White Hairs not being innately compassionate if I use upbringing as an explanation for their behavior.
Thanks in advance. And if there’s anything wrong with my question or if more information is needed to answer it, please let me know.

A:

Evolutionary Benifit
Every single thing we do behaviorally is due to some benefit or quirk of evolution. In our species altruism and empathy exist because its a survival trait. Helping another member of your species for no personal gain is a highly valuable group survival behavior that benefits a species. In order for this to happen in a conscious and sentient being one needs to posess empathy, which is just our word for an instict that allows us to detect, understand, and relate to another’s suffering. In humans it was such an integral part of our species survival that this instict evolved to become so strong that we will often empathize and perform altruistic acts for members of OTHER species. That’s where the quirk part comes in. Our empathetic drive is so strong that we see a kitten stuck in a tree, or a dog with a lame paw and we immiediatly humanize them and feel compelled to help. Saving that kitten or giving that lame dog some help by removing the thorn from his paw serves absolutley no benifit to our species survival, but we do it anyways.
Conclusion:
Your aliens simply evolved in an environment that required an even higher level of this drive to survive. If humans can look at something as different from us as a dog and feel empathy and an altrustic motivation to help, then you bet yer ass these aliens that look like a pallette swapped version of us with pointy ears is going to begin applying that same survival trait to us. In their instictive cave-brain we are little more than just stubby eared pink or brown versions of themselves. Thier evolutionary history was full of incredibly dangerous threats that could only be overcome by working together and communicating to a very high degree. Perhaps thier planet used to posess a terrifying number of nearly sentient pack based predators that had to not only be out-thought, but also out-communicated. Empathy and altruism are two very powerful tools for such a species to develop. 
psychopathy is a survival trait too
The thing is, evolution tries multiple strategies at once. In humans, a small subset of us (about 1%) are psychopaths. This, too, is an evolutionary stratedgy. If you do not posess empathy (basic definition of a psychopath) then you quickly realize that others do, and that you can use it to further your own survival goals at the expense of others. Again, we treat this like a disability, but thats just because our science is biased. Our only in-depth studies of diagnosed psychopaths is on prison populations. The truth is, we are recently finding that psychopaths are more likley to become firefighters, police, soldiers, doctors, or end up in leadership positions. Psychopaths tend to be fearless, and not only that, but capable of making the tough life or death decisions required to lead in a crisis without being crippled by guilt, which they also do not feel. We are finding in new studies that just because a psychopath posesses no empathy doesnt mean they are not capable of posessing morals. Individuals like this were probably invaluable to early man's survival.
The small number of aliens which do not possess extreme empathy and altruism are just that species version of psychopaths. A species can't survive if they are all so lovey-dovey that they can't even bring themselves to kill an enemy. These individuals are relied upon by thier species to fullfill functions that require a more direct and violent approach. They are this race's version of psychopaths, and they form the backbone of it's military, emergency responsponders, and various other functions which would be too morally paralysing for the average individual to handle. 

A:

Maybe the White Hair civilization is led by a small cabal who has instituted some kind of eugenics program centuries ago, aimed at breeding a docile, easily-controlled populace.
They could still be in charge now, or perhaps they too ended up docile; such high empathy would be an attractive trait, so many of the ruling class may end up mating with the docile proles.
So either the eugenicist rulers are still secretly in charge now, or they fell to their own breeding program.  Either way seems pretty ripe for narratives.
Although to be fair, in regards to this statement: “Oh, the White Hairs? It’s the chemicals in their brains that makes them like that. It’s not like they have a choice in the matter”, in some sense that will be true no matter what.  It's even true of us right now.  The chemicals in our brains make us how we are.

A:

The squid people in Charles Stross' Neptune's Brood are perhaps a little like this. A brief quote from one of the characters who lives with them:

They hacked their mirror neurons. And the uncinate fasciculus, whatever that is. There are no sociopaths among them: Everyone has an enhanced empathic sense. [snip]
The fixed a lot of what's wrong with our basic cognitive model. Made themselves over as new communist squid-folk. Yes, they're still individuals, but the border between seld and olther is thinner. And they don't hate. They own property but they don't have strong social hierarchies [snip] - they're instinctive mutualists. [snip]
They get a pleasure-reward for making other people happy. Even an abstraction of other peopole. Isn't that freaky?

For various reasons, the progenitors to the squids (who were either humans, or human-modelled AI, the story never says) wanted to do a bunch of things in a hazardous environment, and decided that tribal tendencies and social hierarchies were hinderances in that enivronment. So they got rid of them, and remodelled their minds and bodies to better suit their needs.
The squids removed sociopathy from their new species, but perhaps it didn't work so well for the White Hairs, because you want some of them to be "defective", at least from the point of view of their peers.
That presents a number of interesting possibilities, including:

The White Hairs modified themselves, seeing the pointless infighting and status-seeking associated with tribal behaviour to be a serious detriment. This is an ongoing project. Perhaps "defective" children are detected early in the education or health system, and they are then corrected... such children would otherwise be a danger to others and would be unhappy in themselves, so healing them is clearly the best thing for all concerned. The cleverest ones, or those with the mildest symptoms might slip through the cracks.
The White Hairs used to be a more humanlike warlike and aggressive species, but were "fixed" after starting a fight with the wrong opponents. The defective ones are in fact the original unmodified species. They may or may not know this.
The defects are the results of external tampering. Someone has decided that having members of a species known to be kind to a fault, but who are actually as hate-filled and grasping as everyone else would be a very valuable tool. Maybe some of these changelings escaped into the wild, or maybe the program got interrupted. Maybe they're cuckoos, sent to be raised by the people they'll grow up pretending to be.

“Oh, the White Hairs? It’s the chemicals in their brains that makes them like that. It’s not like they have a choice in the matter”

Sure, some people might think this, but its only because of the chemicals in their brains that make them think that. It isn't like they have a choice in the matter. Just like that whole out-group-exclusion thing that leads to all sorts of pointless bigotry, hatred and conflict. It isn't really their fault.
Of course, those people might think that they are the sane ones, and their behaviour isn't somehow held in thrall to ancient biological programming. That's ok though, because the White Hairs have an outreach program. There comes a point where leaving a person as they are becomes unambiguously the worst thing to do, because the damage they will do to themselves and others exceeds the perceived benefits of letting them be.
Barbarian species might kill, or imprison such people. As if that will help! It’s not like they have a choice in the matter. Its ok though. We know how your brain works. We can fix the bits that are causing you pain and suffering, and making you pass this on to others. You'll thank us when we've healed you; no-one has ever been unhappy with the end results.

