In the Deathly Hallows, at first, Kreacher is very rude to the three, but after a certain occasion, he becomes a lot better. What exactly made him better? Was it the false locket, the revenge on Mundungus Fletcher or maybe Harry's promise to fulfill Kreacher's promise to Regulus? Did Kreacher actually start to like Harry Potter and his friends? Or maybe he was serving them better to return the favor?
He became nicer after being presented with Regulus’s locket.
While he was happy that Harry wanted to finish the mission Regulus had given him as well, Kreacher seems to be most “affected” by being given Regulus’s locket and being considered worthy enough to have it.
“Kreacher, I’d, er, like you to have this,’ he said, pressing the locket into the elf’s hand. ‘This belonged to Regulus and I’m sure he’d want you to have it as a token of gratitude for what you –’
‘Overkill, mate,’ said Ron, as the elf took one look at the locket, let out a howl of shock and misery and threw himself back on to the ground.
It took them nearly half an hour to calm down Kreacher, who was so overcome to be presented with a Black family heirloom for his very own that he was too weak at the knees to stand properly. When finally he was able to totter a few steps, they all accompanied him to his cupboard, watched him tuck up the locket safely in his dirty blankets, and assured him that they would make its protection their first priority while he was away. He then made two low bows to Harry and Ron, and even gave a funny little spasm in Hermione’s direction that might have been an attempt at a respectful salute, before Disapparating with the usual loud crack.”
- Harry Potter and the Deathly Hallows, Chapter 12 (Kreacher’s Tale)
Harry also attributed the change in Kreacher’s behavior to being given the locket.
“He began to spoon soup into his mouth. The quality of Kreacher’s cooking had improved dramatically ever since he had been given Regulus’s locket: today’s French onion was as good as Harry had ever tasted.”
- Harry Potter and the Deathly Hallows, Chapter 12 (Magic is Might)