I think the different reactions are always interesting psychologically ...Friends of mine do that duck and run, or awkward chat, I'd assume there are some insecurities, resentments, and basic conversational skill defects. My closest friends are almost all friends I met in high school, and we are all still close, obviously there are degrees, and some moved out of SD, or the state/country. There are dozens of 'friends' and people I knew, even from middle school, I'd love to see or run into. I sometimes don't remember people, if they come up, at Nordstrom once, an employee was saying she went to HS with me, and I knew her name/who she was, but I didn't really have any recollection of her, beyond that.
At heart, I am an introvert, but I know/knew a lot of people. I also have a good memory, so more often then not, I can remember exactly what class, or specific memory I have, and I am curious to what I may have forgotten. I'll catch up with someone for a few minutes or longer, if it's someone I have an interest in, or remember well. I remember at my reunion 2 years ago, a few good friends and I were taking the elevator up, and before we got in, and even before the reunion, these guys were acting like they were still 'cool' and other people were 'nerds', as if any of that matters now. In the elevator there's this nerdy, shy ass kid and his girlfriend, clearly grown up, but still the same... and NONE of them say a word, really immature, we are obviously going to the same place, and I reach out, shake his hand, 'Hey your name's ____right'? We had spanish together, I think. How are you'? Simple as that. I was embarrassed as fukk no one did that. Just basic manners and decency.
And really, in the 'real world' those three friends of mine aren't shyt compared to that nerdy kid, I'm sure he's a CPA, professional sort. (edit: I think dude is actually a neurologist) Those guys are bartenders, never got a degree, living at moms, etc etc. If you are acting like you are cool, because you were in high school, chances are you aren't.