So, probably nostalgic melancholy. That's the first thing that popped into my head.

Every once in a while some stimulus, usually a song or a picture, takes me right back to the crazy times I used to have with my crew in high school, or to quiet game nights with Tchaikovsky and my family before we stopped being able to spend time with each other, or just reminds me of some specific moment. Since I'm currently a happy person with an upbeat outlook, these memories don't inspire me to compare my life now with what my life was then, so the feeling is wonderful (which it might not be if my life had gone downhill or something) -- I can smile through the tears, as it were, without too much trouble. But the wonderful memories come with the reminder that I'll never recapture certain moments in my life again, and that's a really strange feeling. Too, there's a certain tendency to revel in the melancholy; an orgiastic indulgence in self-righteous masochism. If I see one old photo that reminds me of a great memory, I'll probably have a look through the rest of the album to encourage the feeling to last. Etc.

They say it's scientifically proven that humans respond more to sad songs than to happy ones -- I believe it. What's complicated about this emotional set is not what I'm actually feeling; it's that the best part in some ways is not remembering the happy memories, rather it's the sadness that comes from knowing they're in the past.

Hope that's a decent enough answer to your great question. I try to stay mostly personality-superficial on hubski, but sometimes a particularly good prompt inspires me to share.