If you go over the work of an author (be it novel or lyrics) your going to find some of what they've experienced, what they believe, how they've been hurt, what they want to do.....
I find it an interesting study to read multiple works by the same writer and then research their life and see the similarities. Its much more obvious with some authors then others, but normally you can see something there.
Its not a blanket statement, but I believe many authors and song writers are more reserved; the type of people who really don't go around telling people how they feel or who tell secrets very freely. Maybe that's why some of them need to write- to get things out. (That's of course only one reason among many.)
Besides the fact that they open themselves to sometimes vicious criticism on their work, writers open themselves up to speculation and a more personal type of criticism. All writing that works has truth in it and sometimes its painful.
Just some thoughts... :-)