We all exist in the first person. So if you write in the first person you're automatically so much closer to life as it's really lived. With a first-person narrator you know why the story is being told. It gives you immediate access to motivation. The story in third person has to come out of the blue from some mysterious point the author has decided on. I always have a slight sense of the author taking a superior stance.
Not sure if this makes me feel better or worse. On one hand, since first person is largely what I've instinctively wanted to write so far, I feel vindicated. It's not that I can't do anything else, but sometimes it just feels right.
On the other hand, I'm a bit ashamed to admit that I thought if you were going to be a proper grown-up writer you had to do third person. In fact, I've been told that quite clearly.
So I'll ignore that bollocks from now on and trust my instincts. It depends on the demands of the story and character, and I've always known that deep down.
But on the third hand, should there be such a thing, does that mean I need to rewrite the entire story?