I think a big part is that he treats everyone with respect. He doesn't have prejudice or pride or anything, so whether its a senior officer, a 'lower decks' officer or a civilian, he listens to them and treats them as equals. He's even polite and civil to his captor in The Most Toys!
His childlike innocence and na´vety, combined with his massive strength and intelligence, make him kind of the type of person the human race would aspire to be in an idealistic, optimistic future, where you can afford to be innocent and naive because there's no greed or selfishness or people taking advantage of you.
that's a good point about the "treats everyone with respect" without regards for things like rank or status. You're right, you got politeness and civility no matter who are what you were.