Spoilers ahead for Avengers: Infinity War! You've been warned!
Though Marvel Cinematic Universe fans may love to debate on the topic of which Avenger is truly the most powerful, it's hard to deny that Hulk (Mark Ruffalo) has the rawest strength. That said, the character gets his ass handed to him by Thanos (Josh Brolin) in the opening minutes of Avengers: Infinity War and the big guy spends the bulk of the film hidden away inside of Bruce Banner, refusing to come out. Fans have speculated about the reasons why Hulk doesn't emerge, and Infinity War director Joe Russo opened up in a recent appearance on the Happy Sad Confused podcast and explained that it has to do with Hulk's refusal to protect Banner anymore. Russo said:
I think people have interpreted it as Hulk's scared. I mean, certainly, that's not a -- I don't know that the Hulk is ever... he's had his ass kicked before, and he loves a good fight. But I think that it's really reflective of the journey from Ragnarok. Is that these two characters are constantly in conflict with each other over control. And I think that if the Hulk were to say why, it'd be [that] Banner only wants Hulk for fighting. I think he's had enough of saving Banner's ass.
So, it sounds like there is an element of fear, as Hulk did take a serious beating at the hands of Thanos. That said, Hulk is still a fighter and it's not purely a matter of him fearing Thanos -- otherwise, it wouldn't necessarily make sense in the context of his battles with Surter (Clancy Brown) and Abomination (Tim Roth). It's not just fear. As Joe Russo laid out during his appearance on the Happy Sad Confused podcast, Hulk has finally decided that he doesn't want to bend over backward to protect Banner anymore -- further emphasizing the growing rift between them.
As a result of Hulk's refusal to protect Banner in Avengers: Infinity War, Banner ultimately has to learn to protect himself in combat. He does this by taking control of the Hulkbuster armor, and he's one of the few Avengers team members who get to take down a member of Thanos' Black Order, in this case, Cull Obsidian.
We will have to wait and see how Hulk's attitude towards Bruce Banner and heroics evolves over the course of the upcoming arc. Though the green Avenger hasn't fronted a film since 2008's The Incredible Hulk, there is one long story playing out, and we're right in the middle of it. Specifically, Mark Ruffalo and Kevin Feige have a trilogy planned for Hulk that will follow the character through Thor: Ragnarok and Avengers: Infinity War, and conclude with Avengers 4. With one movie left to round out that story, Hulk and Banner may only grow further apart.