There are some great songs about being happy:

Happy by travis,

Cherub Rock or Today by the smashing pumpkins (if you listen to them carefully, they're possibly the most happy songs you'll hear)

I'm so happy you failed by Laptop (well it is happy ;))

8 ball or Bucoup Fish by underworld

Pure or marvelous by Lightning seeds.

most of the beach boys stuff.

god there's loads... that just scanning through the first file of mp3 collection..

About pain.... I dont know, when ever I'm down, I stop being creative... I dont know why, I lose the drive to write... but being happy seems to make me more patient and willing to sit down and work with something till 4am... hehe..
Playing with other poeple is great for ideas, becuase I play mostly on my own, and jam along to tracks i've made, working with someone with a completely different style really get the ideas flowing...
Being a musicain is kinda of alot like being an actor, you've gotta convey emotion, but you dont have to necicerly be expereincing it your self, and all the matter is what you put across... "stop whispering" by radiohead is a fantastic sad song, about the hurt of being left... but it never happened to tom york, he just kinda put him self in a hypothetical position and wondered how he'd react...