We tend associate things with people we know, purely based on personality. There's no psychological reason I've heard for why we do this. I do know it's part of human nature to categorise the items and people in our lives.
I will always associate "New Slang" by The Shins with Julian. It comes from a mix CD he made for me when we were dating. It also comes from seeing
Garden State and identifying with the themes from the film. I was listening to the song while driving to school and was reminded of Julian and those wonderfully good feelings I had associated with him.
It's hard to dissociate songs from people, and I've tried to do so, mostly after the relationship ended with Julian. It didn't work. I came to the realisation that even though that things didn't happen the way I wanted, it was still good to have the memories of the nice things.
The odd thing is that Julian's initiated conversation after some time. Those associations I've made are still around and I won't dismiss them.