Argh! That drove me crazy. Kreacher went from a person to a lower form of intelligence in between books. Hermione was nice to him and Kreacher said, "Why is the Mudblood talking to me?" Suddenly everything that looked like Kreacher acting like a Mrs. Danvers-type servant it turns out all they have to do is be nice to him. So why on earth didn't he like Hermione before??

Also it's infuriating the way this is pretty much the only place in the book we get the idea that nasty people could become your friend with some effort. you don't have to put in the effort with actual people, there's this ridiculous creature created specifically so you can do it easily and pat yourself on the back.