I got to really thinking about the lyrics to this song and although the obvious interpretations are either the war in Iraq or a relationship with a girl, I think the song may be about suicide. Before you dismiss the idea as ridiculous, listen to some of my interpretations on the lyrics.
'There's another world inside of me that you may never see"- A lot of people who are depressed or suicidal don't like to talk about their deepest feelings and what is making them feel the way they do.