I didn't really take it as ambivalence over being a hunter--I thought the tension in this episode was over Sam as a leader and teacher. Mary talks about watching him come into his own over the last few weeks when obviously he's been hunting a lot longer than that. Bobby never criticizes his hunting skills, just his command decisions.

It's being in a position of authority and being responsible for others that he may or may not be born to do. I think some of his ambivalence is simple insecurity--he doesn't feel like he's earned the praise and he's not sure how to internalize it. He's probably got some impostor syndrome going on. He became the leader while his attention was elsewhere--on the hunt for Dean, on what Dean was going through--and so he probably didn't consciously examine the decisions that made him the leader that much. He's now settled into having Dean back and that means he can take a breath and look around at what his life's become. I don't think he's miserable about it by any means. He seems at his most proud and content when he's doing the teaching part of the job, which makes sense as that's when he's making a positive difference, improving on stuff and not just running damage control--which has been something he's wanted for a while.