If you don't put any of yourself into a song you write, it is much harder to feel any passion about it. It is not necessarily about yourself but conveys the way you feel about something. If you don't put something, whether from your own sense of humour, your experiences, your thoughts etc...which bit of it is yours? They are just words and a tune.
If it is telling someone else's story it will still have something of you in it. I think.