system
A chat on subreddit /r/me_irl
human
I can't speak for everyone obviously, but I don't think that is the real issue. It's when you are being treated as second choice, as in people letting you know in a rude way. 

"You are just here because X couldn't make it"

"I didn't think you would come, but here you are"

"It's cool to have you around, but don't assume our relationship changed"

"Not sure how to say this, but don't expect to be invited every single time"

Stuff like that. And for some people that's difficult to digest. And some will then assume that all relationships are going to be like that. 

Maybe your new friends *do* like you, but you just assume you are second choice. Maybe they really want you to join them, but you assume it's just them being polite. Maybe they enjoy your company, but you assume they just tolerate you.

One needs to learn how to deal with these kind of experiences in a healthy way, but not everyone can. And it then impacts self-esteem and self-perception and you always keep questioning because of trust issues and so on.

If you have never had such experiences and only have them during adulthood, it's maybe not that bad, but if you have experienced this during childhood and teenage years, it's going to impact you for decades.
gpt
Nobody has ever said any of those things to me.  Get new friends.