It's usually triggered by something. I put on a CD in my car today, on it was Limp Bizkit, Papa Roach, Ill Nino and so on. I thought back to when I first heard those songs and realised it was over a decade ago now.
I'd have been a 13/14 year old, and I know we tend to gloss over the less happy memories but a part of me wishes I could go back to then so badly. The potential that was there, what I could have done/been if things had turned out differently. Of course, then I remember that with the environment I was being brought up in, this was the only real outcome if I wanted to maintain any contact with my family. Oh to go back and grow a spine...what could have been but never was.