There are SPOILERS for Infinity War below, so read ahead at your own risk. For those who have seen Infinity War, the Hulk fights Thanos in the very beginning of the movie, only to get his butt handed to him by the Mad Titan. Heimdall than sends Hulk to Earth to warn Doctor Strange of the impending arrival of Thanos and his Black Order. That's the end of Hulk in the movie though. He's teased throughout the rest of the movie, but he does not want to come out and play with Thanos again. The Russo Brothers have now revealed why Hulk didn't come back.
Bruce Banner spends a lot of his screen time in Infinity War at odds with Hulk and many fans of the Marvel Cinematic Universe believe that it's because he's too scared of Thanos after their first meeting. Bruce Banner is visually shaken when he confronts Doctor Strange and later Tony Stark. Ebony Maw and Cull Obsidian pay the group a visit in New York to obtain the Time Stone and Banner cannot get the Hulk to come out and fight, which means that Banner has to sit the fight out.
Joe Russo discussed the reasoning behind having Hulk sit the majority of Infinity War out. The director admitted that it was not because he was afraid after confronting Thanos on the Asgardian refugee ship. Instead, it was for a completely different, but very important reason. Russo had this to say.
"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."
It seems highly logical that the Hulk had some time to think about his relationship with Bruce Banner after the events of Age of Ultron and leading up to Ragnarok. It seems that the Hulk has grown resentful in the years that he was able to take over, but what does that mean for Avengers 4? Bruce Banner was able to get into the Hulkbuster suit and serve his purpose on the battlefield in Infinity War, but Banner and Hulk definitely have some issues to figure out.
Now that we know that Hulk wasn't too scared to fight Thanos in Infinity War, that means that he and Bruce Banner will have to settle their differences during Avengers 4. Mark Ruffalo has been spotted on the set of Avengers 4 with the motion capture gear on, so we know that the Hulk will be returning, but just how willing of a participant will he be this time around? We'll just have to wait and see, since Avengers 4 doesn't hit theaters for another year. You can read more of the interview with Joe Russo over at Cinemeblend.