Novice
Being new to something does not necessarily equate to being unskilled at it.
The first time Dean went shooting, he hit every bottle. He can still remember how proud his dad was, how much he smiled as though Dean was impressive. Dean still holds onto that feeling. It's nice to be good at things. Which is why it's scary to try new things.
What if you're no good at it? What if you suck? It's embarrassing. That's what makes Dean hesitant.
But Cas doesn't seem to have that problem, Dean notices. He approaches every new thing with a clinical appraisal and a surprising passion that Dean did not expect from the stoic soldier. He seems to simply believe, without doubt, that he can and will excel at anything he puts his mind to if he is allowed the practice. It is this mentality, and un-reproachable work ethic, that has him on his knees between Dean's thighs five days in a row.
He intends to conquer this challenge. And though Dean tells him on day one that any way he goes about it will probably do the trick, Castiel aspires to true skill. And Dean cannot help but feel like maybe he's having some sort of extremely vivid wet dream... because the whole dynamic is truly the stuff of fantasy.
Castiel teaches Dean, over the course of one extremely pleasurable week, that all it takes to go from novice to expert is a little courage.