Cats hate change. For example, you bring cat A to the vet and when you get home cat B suddenly hates cat A. This is because 1: you took cat A away and while you were gone cat B looked everywhere for cat A to no avail. The thing is, cat B knows every single place cat A hides. Every single one. And if B can’t find A then A has disappeared and that it not normal and cat B does not like not normal at all. He’s a cat. Not a human. Oh, and nevermind the fact that B saw A in the awful cat carrier. B might remember what happens when you get in the cat carrier, or, he might not. But it probably won’t occur to him to make the connection because again, he’s a cat, not a human. And then there’s problem #2: you bring cat A home and he does NOT smell like cat A. He smells like cats B has never met and that is NOT good. By this point cat B is highly suspicious of what’s going on. Sure, it looks like cat A but it doesn’t smell like cat A, plus cat A was gone and who is to say this is the same cat A? It could easily be a replica. All of which results in cat B deciding to hate cat A who is not cat A as far as B is concerned.
Now, if you have dogs, dog B will exuberantly greet dog A like its been gone forever, and the two will resume their normal relationship as soon as the excitement wears off.