ok new headcanon aquired: i don't think being healed by angel grace is in any way pleasurable actually. I think it is only pleasurable to Dean. And only when it's Cas who is healing him. And it's because grace is the angel essence in the purest form. The warmth and safety Dean feels every time Cas heals him? That's because Cas loves him. Every time Cas infuses Dean with his grace, Cas' love flows thru Dean's veins. Literally.