Set AU-ishly somewhere in Season 4 (no demon blood addictions, Jimmy is gone). Castiel's vessel has started to develop basic needs. His libido seems to think it's a fifteen year old's. Sam helps him control it.
After things have settled with the Leviathans, Cas goes to visit Sam who has been having bad dreams and a phantom pain.
Dean goes missing searching for someone who might be able to heal Sam’s permanently ruined leg. In the course of searching for him, Sam and Castiel fall into a trap and Sam suffers an asthma attack.
Sam is cursed with hair so awesome that anyone who looks upon it cannot resist him. Unfortunately this includes his brother, forcing Sam to go into hiding while Dean figures out a way to break the curse. Alone and not even able to venture outside, Sam's only company is Castiel who is immune to the curse.
Castiel is having trouble finding ways to express his love to Sam. He eventually finds his way to Sam's heart by following the example of a very suave young gentlemen he watches on television.
When Castiel looked into him for his soul and didn't find it, he left a little bit of his grace behind. Now that Sam's soul is back where it belongs, it keeps brushing against the bit of Castiel's grace.