I read somewhere that Kirk, ubermasculine guy or not, represents the feminine aspect in his partnership with Spock; and that's always stuck with me. Spock was a lot more approachable for me after his characterization loosened up enough to show how much was always going on under the surface even when he was being all stoic and so on; but Kirk's fierce attachment to life, ship, crew, and so on is just so much more appealing to me on a visceral level.