Sam's faith is comforting for Dean, it's something he associates with Sam, and he would cling to everything he knows and is familiar with about Sam. Even something as mysterious as faith.
On a side note, I'm not sure Sam's faith is truly faith. I had an interesting conversation about this aspect of Sam some time ago, if I can dug that post, I'll link you.

On a lighter note, Dean can't figure out how to work Sam's iPod.
*lol* Oh, Dean.