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.
And I think that if the Hulk were to say why, it’d be (that) Banner only wants Hulk for fighting.
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.
Joe Russo discussed the reasoning behind having Hulk sit the majority of Infinity War out.