Biological anthropologist Helen Fisher has a pretty perfect description of what it's like to be in love with someone:

Simply put, she says, that person becomes the center of the world. You have an intense craving to be with that person, not just sexually, but emotionally. You can list the things you don't like about them, but all that gets pushed aside and you focus only on what you do like about them.

"It's an obsession," Fisher said in TED Talk called "Why we love, why we cheat."

What's going on biologically, though, is far less romantic, and it explains why we sometimes cheat on those we love.

Romantic love is essentially just elevated activity of the reward hormone dopamine in the brain.

In the TED talk Fisher explains an experiment where she and a team of scientists scanned the brains of people who were in love. The team showed the smitten person a neutral photo and then a photo of their beloved. They recorded which regions of the brain were active while the person gazed at the photo of their partner.

The researchers found that one of the most important brain regions that became active when each person looked at a photo of their partner is the reward system — the same brain area that lights up when a person takes cocaine or has an orgasm.

That means that "romantic love is not an emotion, it's a drive," Fisher said. "And in fact, I think it's more powerful than the sex drive."

Many other studies have found the same thing: love operates as a motivation and reward system in the brain. So, if love is rewarding, what drives us to cheat on people we fall in love with?

The problem is that romantic love isn't the only brain system that is activated when we fall for someone. There are actually three brain systems related to love, Fisher explained.

There's the sex drive, which is like an "intolerable neural itch," to get us out searching for a range of partners to help pass on our genes. There's romantic love, which helps us focus our mating energy on one person. And then there's attachment, the calm and security we feel with a long-term partner so we can raise children with them as a team.

However, those three brain systems, sex drive, romantic love, and attachment, aren't always connected to each other.

So it's possible to feel deep attachment to a long-term partner at the same time you feel intense romantic love toward someone else and even also feel sexual attraction toward another person, Fisher said.

"In short, we're capable of loving more than one person at a time," Fisher said.

And that's why, Fisher says, some people may cheat on their partner.

It's why someone can lay in bed at night thinking about deep feelings of attachment to one person and swing to thoughts of romantic love for another person.

"It's as if there's a committee meeting going on inside your head as you try to decide what to do," Fisher said. "I don't think honestly that we're an animal that was built to be happy — we're an animal that was built to reproduce. I think the happiness we find, we make."

This all sounds like a cynical take on love, but Fisher says that, despite all these straightforward and unavoidable biological processes, there's still mystery and "magic to it."

Watch the TED Talk here: