i think he wrote it about his girlfriend...(obviously). nocemisions is right cuz the lyrics are random and in a way confusing. this is a great song though!
andydog1 is actually right. in a biography, he said when he was writing songs, people asked why he wasn't writing them about his then-girlfriend, Tracy. when his friends asked him what it was about, he simply said, "About a girl," so the name stuck...
I think that there's a double meaning: a girl is also Heroin. "but I can't see you every night" was his lament about trying to avoid addiction, which sadly went badly for him.