i'm not sure i really have a go-to song when i'm happy. i don't go, "school's closed, and some dude just gave me twenty bucks for a dead bird. today's a good day. next, i think i'll go turn on some music." the opposite happens...i'm having a neutral day or even a sucky one, i turn some music on to fill in some perceived empty space or to give me some emotional support, and the day gets better. but, i can't think of the last time that happiness was what led me to turn on music.
however, i do have an answer for a song i associate with loudly singing when i'm happy [and possibly somewhat inebriated]: "Kenesaw Mountain Landis" by Jonathan Coulton.