I think the best songs come from unsent letters or poems.....it's really all about what you felt at that moment. Even if your venting and you may not feel that way later, sometimes down the road you can read whatever you wrote and just relate or see how much you've grown from it.
Example: For some reason when I just need to think, I play (u2 with or without you)Depending on whats on my mind this song never has the same meaning.