[…] Because Sam and Dean are eachothers only angels, because they can only hear God’s message of faith, hope and love from eachother. One without the other is less than human. Angels are vulnerable to falling, and Sam and Dean are no exception.
Dean has no faith in angels because he says he has never seen one. If he wants to see an angel with his “own two eyes” all he needs to do is look at his brother. Or look in a mirror.
From the Supernatural book “In the Hunt” pg 63 (published just after season 3 finished)