I usually start making a song when I wish to try out something new, I've come up with or read online: key signature/structure/effect/etc. Innovation is what usually drives me.
As for the emotional constituent, I tend to come up with various moods, from stupid happy to aggravating despair, which reflects my own constant mood swings, I guess. It is nigh impossible for me to single out one motive that binds this eclectic library. If there is one thing, that most of it seems to have, it is the feeling of emergence, hurry and unresolved tension. This is probably because, when I started taking interest in making music, the songs, I appreciated were limited to techno/trance genres,
Oh, and you also could add escape from the miserable reality to the motives list. Or was it motivation?