I think it's a simple answer, but with lots of variations of uniqueness. i think alot of it comes down to the conditioning of personalities they were always with growing up, upbringing. personal traits like personality, quirks, swagger.
person A: grows up is mild mannered and quiet, perhaps moody and somewhat defeatist, but can be silly, fun, chatty with friends, has a group of friends, 4-5 close friend with many other acquantences, something happens, they get bullied, maybe two of those friends have a big argument, maybe there is some bullying somewhere, they then become more reserved, switch off, become very independent and fly solo instead. Hang on to the belief that they weren't wanted or that it was time to disengage from the group. This becomes the "norm" after a while. Really needs some friends to give them a boost of self-esteem though. To remind them of their importance. Without that, more likely to stick to a cycle of "safety". Wait for someone else to befriend you, which may never happen unless you take a risk.
person B: Grows up, is loud, unselfconscious, has a fearless approach to making friends. has some coolness about them that people like so continues to make friends, has some problems but never enough that it effects personality or belief entirely. So progresses. Will dictate asking friends to go places, is generally the one who make the phone calls and shows interest in topics, conversation. More likely to always have a group of friends in some form throughout life.