In the opening scene of Avengers: Infinity War, fans saw Hulk get the beatdown of a lifetime from Thanos before being transported to Earth. Subsequently, the Jade Giant refused to come back out to join the Avengers, leading fans to believe he was afraid of the Mad Titan.
However, courtesy of Hulk actor Mark Ruffalo, we now know the Green Goliath's greatest fear, and no, it isn't Thanos -- it's someone director Joss Whedon discussed years before with Ruffalo himself: Bruce Banner.
“There’s a conflict brewing between Banner and Hulk that’s been going on since the beginning," Ruffalo said on The Marvelists: The MCU Podcast with Peter Melnick and Eddie Wilson. "And it all came out of sort of this question that Joss Whedon asked me when Hulk gets the fairy dust from Scarlet Witch sprinkled on him in [Avengers: Age of Ultron], Joss asked ‘What is Hulk afraid of?’”
“And that was a hard thing for me to answer because what is the strongest, fiercest thing in the universe afraid of? And then I realized it’s Banner," he continued. "The only thing Hulk is afraid of is Banner and so that’s become something that we’ve been riffing on since [Age of Ultron]. We played with it a lot in [Thor:] Ragnarok and it continues to be teased out in [Avengers:] Infinity War and [Avengers 4].”
This may come as a surprise, given that Hulk looks down on the puny human, but theorists believe he fears Banner because he's scared of being used as the Avengers' muscle once again, ergo why he doesn't want to come out anymore. Of course, given Banner knows the "big guy" inside out, there may be something more to this revelation, but we'll just have to wait until Avengers 4 to find out more.