Because all that earnestness? All that puppy-like enthusiasm? It only matures along with Chris, evolves with him, turning him into a kind, thinking, great man. One who also dates models, and Zach can’t help but compare and turn it into some sort of competition. You see, he was the one who fucked up, and he was the one who did the hurting, so naturally he can’t help turning it all around to make himself feel better, until it’s all somehow Chris’s fault in a twisted way. And it’s petty and wrong, but he can’t help it, and it ultimately damages their friendship too. He can feel Chris pull away, which is a good thing because he can now legitimately blame him. At least in Zach’s eyes.
See, now I want to stop your reply at “can’t help but compare.” And then make Zach realize his mistake, and have it not be too late, and have Chris say, “never fucking do that to me again.” and have it all work out and be part of the story about how Zach learned to value Chris.
I basically suck at sad headcanon. And by the way, my version is almost exactly what happened between my husband and I 21 years ago. Made us stronger.