The Russos actually gave a different answer to what happened. It was something about Hulk being tired of being used to solve all of Banner’s problems.
Now where the explanation for that motivation came from, what it had to do with anything related to the Hulk in the other movies, and why it all of a sudden became a problem then and there are never explained. And it’s also never explained how Banner merging with Hulk solves the problem.
But that’s always been the problem with these movies: they set stuff up, but they never really follow through.
It had everything to do with Hulk in other movies. From being hunted down by Ross and Banner himself (trying to commit suicide) to Avengers trying to lock him up until they need his help, to Tony inventing tech specifically to take him down to him being so ashamed at how people were afraid of him that he hopped into the Quinjet and left the planet entirely.
Then he ends up on Sakaar where he's not only worshiped as a god for doing all of the things that he was shamed and hunted for on earth.. He also gets 2 years of full control to be out and about, experience, learn, form opinions, understand friendship, ect.
He remembers that he was hated on earth. He also remembers that The Avengers were "Banner's friends" not his with the closest thing to having bond with any of them was with Thor. He didn't want to go back there, he didn't want to help Banner or his 'friends' out of a jam and then be told to go sit in a corner. The Merger solves the problem, at least temporarily because it makes a compromise between the two personalities. Banner gets to 'steer' and retain his intelligence without being weak, Hulk gets to experience life and be adored and feel included, it's why he was so thirsty for signing autographs, selfies and doing good deeds, Hulk get's to be loved instead of feared, like he was on Sakaar.