I think on my part it's partly I just never really liked 99% of my highschool class (especially my 2nd highschool). And partly the fact that I run into these folks randomly everyone couple months or so, or their siblings who have to tell me how their big bro/sis is doing. I'd have to say most of them have a very rosy picture about that time in our lives or they downplay the shit they did to people with a laugh and "Ha, ha, can you believe we were just kids."
Last one I ran into that did the whole "wasn't it such a good time, to be that age again" I reminded that she and her friends badgered one of our younger classmates about her looks till she [the classmate] attempted suicide and was put away for a couple of years cause they were "just having a little fun", or hey, let's talk about Lisa, you know the girl whose mouth you held opened sprayed full of mace and damaged her lungs cause "Bitch needed to learn her place". Suffice to say, she told me to stop dwelling on the past and that I can't hold what we did as kids against her.
Of course there was the girl who tried to trick me into our class's one reunion. She called up and tried the sexy voice "I'm really looking forward to seeing you again" angle. I recognized the name, knew she was the bitch that ruined one of my friends by claiming he knocked her up (he hadn't, she was screwing another guy) and asked her if she'd do a three way with the wife [we're not into that, but hey shock value]. She hangs up, two days later her mother calls (recognized her mom's voice, she's got one of those annoying voices that you never forget). She started into the usual bullshit about "good times" and her daughter really would like to see me, blah, blah. I just said, "hey how is old doorknob doing?...You know our nickname for your daughter, everyone with $20 gotta turn." Never got called again.