Gavin has said before that he has "crappy hair", so he likes the hat. I've seen his hair, and it looks nice to me. He also has a large forehead.
Below are some pics, check it out. I think Gavin looks great without a hat. He should go hatless more often.
http://degraw.yuku.com/topic/422/t/Hatless-Gavin-Pics.html#.T4-RfFGvKS o
It's Like He Knew That I Am Fragile
He Handled Me Like Glass

• Glass by Gavin DeGraw