I don't know why, precisely, but I find myself somewhat surprised at the large response. I grew up as a musician (keyboards for 12 years) and I know a large number of writers also play musical instruments, but I'm kind of surprised so many of you also have songs or even entire soundtracks for your stories and characters.
In my case, the theme song says something about the story I'm writing, but doesn't generally, tell the story. The story is not about the song in other words. For example, New Divide by Linkin Park has a certain portion that talks about two people looking across what seems like a great divide between them. In my WIP the divide is between where the main character is in the "now" and where he used to be and where he wants to go in the future. It's kind of a story about a hero who lost his way and badly needs to come back and make a difference when his leadership is desperately needed.
Likewise, The Only One by Evanescence has a lyric in it that I relate to a character who's been a hero before and got tired of constantly being the hero. Now, after years in involuntary retirement (enforced by a mind wipe) he's sought out by those who need him to remember who and what he once was. He's caught in a dilemma as he really is "the only one" in this case.
I'm sure others would see the music differently, but that's part of the beauty of music.
I listened to a classical station while writing my first novel btw, so I'm not just a rock fan.