So, Spock vs McCoy is one of the great frenemies of fiction, and you love to see them hate. But I decided to put some thought into what it is that makes them react to each other so… poorly.

Now, in McCoy's case, we have to do some inferring, but I think that we know enough about McCoy's character to be pretty confident about our base assumptions. So we know that McCoy is divorced, and in that divorce he lost custody of his daughter. We don't really know much about what his relationship was like, but given that this is Bones we're talking about I have to imagine that he would have loved his wife with his entire heart and soul. Maybe he could've had a hard time getting away from work to spend time with her or something, but he definitely would have loved her with everything he had. And I furthermore think it extremely likely that he was the one to call it off. The man is loyal to a fault and would never give up on a relationship like that.

She also must have had something on him in order to get 'the whole planet in the divorce' (there's no indication that this is a timeline divergence from TOS to AOS, so I assume it's true in both timelines), and it seems unlikely that he would purposefully enter into proceedings where he was likely to lose that badly. (This situation will also be addressed in another chapter.) I therefore assume she was obviously the one to call it off.

What this means is that McCoy gave away his whole heart to someone who wound up not loving him back, even to the extent of taking away his daughter. That would be traumatizing and I don't imagine he'd be too happy to go getting attached to someone who, from his perspective, might not even be capable of loving him back.

This probably explains the denial Bones is in concerning Spock's lack of emotion. He doesn't want to believe Spock can have emotions and he doesn't want to be attached to the Vulcan, either. See The Immunity Syndrom, where Spock wants to be the one to go commit suicide against the space amoeba, and McCoy accuses Spock of being motivated by wanting to prevent Bones from making scientific breakthroughs that would make him famous. As if Spock woke up that morning and went - Well, I may die today: I may perish in the cold vacuum of space far from the entire Enterprise, but by Surak Dr McCoy will not be published in a scientific journal today!

Clearly, this is a ridiculous accusation, too ridiculous for even McCoy to make it seriously. He must be trying to come up with an explanation - any explanation - that prevents Spock from caring about him or the rest of the crew. McCoy can't even bring himself to wish Spock luck while he can be heard - the guy's having serious emotional conflict about his attachment to Spock.

Once McCoy makes peace with the fact that Spock has emotions, he's constantly pushing Spock to express them in words. He wants to know for sure, beyond a shadow of a doubt, that Spock is capable of returning his feelings before he really commits to care about him. Unfortunately, McCoy is incapable of not caring about people, and his emotions just run off and care about Spock whether he wants to or not, which scares him. He doesn't want to be this attached to someone he doesn't understand, someone who would actively deny any emotional attachment, but he is attached.

And the best way McCoy knows of dealing with fear is turning it into anger.

So McCoy is afraid of loving someone who won't love him back again.

Meanwhile, Spock meets McCoy, who rags on him for being a Vulcan, and proceeds to go 'Oh look, another racist, I know how to deal with this one' and proceeds to do the Vulcan thing and block out his emotions about it. Even when he realizes McCoy actually does care about him, he still responds to things that make him uncomfortable by shutting down his emotions.

He also responds to people trying to force emotions out of him by shutting down his emotions, because that was the done thing on Vulcan.

This is an unfortunate vicious circle, because it means that Spock shuts down his emotions which freaks McCoy out. When McCoy freaks out, he lashes out at Spock. When McCoy lashes out Spock, Spock shuts down, which freaks McCoy out so he lashes out at Spock so Spock shuts down…

You see the issue here.