They respected each other, and they got along pretty well when Kirk was around because they both considered him a friend. But for all the times they helped each other out, they always played it like it was the right thing to do, duty, etc., but we were meant to understand that they really liked each other and just weren't going to say it. I really wasn't sure if they liked each other. If it was a friendship, it was a friendship borne of familiarity and respect, and in an odd way, I guess you could call it affection. But really liking each other? I just never quite bought it. That's why STIII felt so big to me. After being inside each other's heads, literally, for a while, they finally reached a point where they understood each other. Singing around the campfire together at the end of STV, that felt like three friends. Real friends.
Maybe I'm looking for an arc that really isn't there, and even finding it. But I never felt like they were real friends until the movies.