Which song makes you happy every time you hear it, and why?
New Order's Regret
. Actually, nearly everything by New Order, with the exception of some of the stuff on their later albums, makes me instantly, inexplicably, happy. Sumner, Hook, Gilbert, and Morris appear to have found a direct connection to my limbic system. (Post-New Order spinoffs Electronic and Bad Lieutenant still manage to hit the same neurons occasionally, but Joy Division doesn't. Which ought to make it clear that this is an entirely separate phenomenon from the pure aesthetic appreciation of the music.)
I chose "Regret" in particular because it was the song I listened to over and over again while I was in the process of escaping graduate school, probably because it captured the mixture of hope and regret I felt at the time. Now, hearing it always reminds me that it is possible to completely screw up something that you thought was really important to you, and still have things turn out better than you'd have ever imagined.