My wife hates that I'm so nostalgic. Barry Hannah wrote along the lines of people in the South become nostalgic at age 11. If so, I'm definitely a Southerner (just lacking the accent and other negative connotations).
We leave behind so many things in our lives -- things, places and people we didn't mean to leave behind but from whom our life's path led us away.
If you're like me and a bit depressive in general, you know how difficult it is to be happy or content, to look forward to things with a positive attitude. You know the difficulty of making close, in-person friends. And you know how hard it is to leave those things behind and go back into your lonely darkness.
Much of my writing, when it's not topical, is nostalgic. My wife worries that I idealize 20 years ago and denigrate my current life with her. That's not the case. Twenty years ago I wrote nonstop about what was going on in my life (it was much more exciting when I was younger). Nowadays, there's not much to say about daily life (and even less that my wife would be comfortable with me writing and publishing) beyond work, reading and sleep.
Hell, if we're honest, I likely wrote most of that stuff with the expectation that I would go back and relive parts of those times in my mind when reading them in the future. The benefit of have a 20-year old online journal/blog.