I think it kinda comes with ASD
It's the lack of ability to connect for most, and it only gets worse if you're thinking about the "why's" and "hows" in regards to human behaviour. A lot of stuff doesn't make sense to me, but people tell me it's usually a bit of a "emotional" thing. People indulding in smalltalk, laughing for (what to me seems) no good reason. It's something I can't force on myself.
I don't know if I could claim I don't fit in with "humans"... but I have a really weird feeling when I am forced to mix in. It feels like it's an undercover mission and I kinda failed that exam (and thus am bad at it). In a way I can get along fine by just being "me"... though people in general told me "you're weird". So that's a reason I don't indulge in social interaction for most and just keep by myself. I don't feel any guilt for being me, but I do feel presured into going out of my own way if I want to be social, when I don't have an intrinsic motivation for. I always feel like showing "normal" behaviour is like a job to me, much like someone who can't walk instinctively but has to think and act consciously for each step. And that's really to tiring for me, and I don't even see a benefit for it. People might call it lazy, but I believe that any effort should somehow pay itself off (in any way, wouldn't go as far as material stuff).
As a kid people already tried to talk guilt into me by saying "don't act this weird"... no, it's not weird, it's me, I really feel sorry if it's weird in your frame of reference.
Coinciding with me thinking about behaviour I'm also quite interested in philosophy, which doesn't really contribute in just going with the flow but makes up for much more critical thinking and the reasons of behaviour. Speaking of philosophy, I hear a lot of people argue that my ideas in general aren't ethical, they're sensible... but in general there's a lot of "oh my, you can't do that... you just can't, it doesn't feel right". And I end up in arguments why that is, it apparently is a trait that comes with empathy to say something is "wrong"...
I actually think that I'm in a bubble, not the rest. But the other way around makes sense. No matter how hard I try... it's a f'n effort to talk to people (not by means of social anxiety)... but sometimes it seems as if I'm speaking a different/foreign language. And that in general gets odd if the people you can't connect with (less and less each day) are people like my parents... I wouldn't be surprised if I was adopted either way (and I've heard people tell me that in the past; I resemble my dad by looks a bit, but for a big part I'm nothing like them in behaviour, values, thoughts and stuff like that, never have)