What I love so much about the Kirk and McCoy dynamic is that Kirk-- you know, God's gift to Star Fleet, space, women and dangling from cliffs and overhangs-- with McCoy can be sort of... normal. Humbled? ;)

That's really what I like about the dynamic. McCoy is old and cranky and has no issue with smacking down his best friend whenever he feels the need. Which tends to be always. So Kirk, in response, tends to be more real around him because he knows there's really not much use in doing otherwise. Oh, he's still an ass sometimes but that's a given.