Warning: This post contains major spoilers for Avengers: Infinity War. If you don’t want to know details yet, click away now and come back here when you do.
The Incredible Hulk might be a giant rage monster who loses control and destroys entire cities, but the Avengers wouldn't have been able to save Earth from Loki and Ultron without him. So it was a big problem when "the other guy" refused to listen to Bruce Banner's beck and call and get involved in the fight against Thanos in Avengers: Infinity War. Hulk smash? No, Hulk scared, after the Mad Titan kicked his giant green ass. But what exactly was the Hulk so scared of?
The Hulk has a long history of being slow on the trigger. We've seen this as early as when former Ed Norton's Bruce Banner jumped from a helicopter over Harlem to face Abomination, only transforming into his super-sized alter ego after crashing through a mound of pavement, and as recently as Mark Ruffalo's Bruce face-planting on the Bifrost Bridge in Thor: Ragnarok. Delays notwithstanding, both cases saw the Hulk eventually arrive to do what he does best. But such was not the case in Infinity War.
At first glance, Hulk's apprehension seemed to be the result of Thanos so easily defeating him on the Asgardian ship at the movie's start. Hulk has taken some shots in the MCU, but he'd never truly met someone who he had no chance against, as he experienced in the ring with Thanos. We know Hulk matured during his two years as the Grandmaster's champion on Sakaar. He learned how to speak, began to develop a real personality, and even started to experience more sophisticated feelings like loneliness, friendship, and loyalty. ("Friends stay!" he screamed at Thor.)
He basically became an oversized toddler, who'd throw a tantrum if he didn't get what he wanted. And like the biggest kid in the class who always runs the playground until one day he gets punched in the nose, he had no mechanism for emotionally dealing with what happened to him when he met Thanos. So he ran and hid and refused to come out. But this isn't so far out of line with what the Hulk has always been about since the day we met him.
When we caught up with Bruce Banner in The Avengers, he had grown distrustful of the Hulk. Despite Tony Stark's urges, Bruce resisted invoking the other guy, calling him dangerous and likening him to an exposed nerve. Meanwhile, Tony insisted that Bruce only need to learn how to control the Hulk, to discover why he was gifted with the terrible privilege of his alter ego, and perhaps even to realize that the Hulk has come into being to save Bruce's life.
We've seen suggestions that this might be a fair way of looking at things, both in the above examples and in Bruce's recollection that he once tried to put a bullet in his mouth only to have "the other guy spit it out." Since then, we've seen Hulk prove instrumental to the safety of Earth by way of his brawn, and we've seen Bruce prove the same by way of his brains. So maybe the Hulk's main purpose is to use his strengths to protect Bruce's.
This even explains why Hulk took over completely for two years on Sakaar. Bruce and Hulk knew he could never really run off with Natasha, that she would never be safe with him. So the green guy fled Sokovia when the battle had been won, and when he ended up on the dangerous world of Sakaar he was better equipped to keep Bruce safe there than the meager Dr. Banner would have been.
But after Thanos took Hulk down so easily, the green guy got scared—for the first time ever, he couldn't protect Bruce. Like a kid who wants to protect their parents but doesn't know how, he hid from the world rather than facing it, much inthe same reason he ran from Sokovia to protect Natasha.
On some level, the Hulk seemed to realized that "brawn" wasn't going to cut it this time around, but perhaps Bruce's "brains" could. He also might have known the Hulk would have been the biggest target on Earth, while no Children of Thanos would bother with Bruce. Because he has the maturity level of a toddler, he didn't know how to process any of that, let alone explain it, and therefore wasn't able to even realize or accept when he was needed.
Bruce Banner and the Hulk's relationship has always been weird and complicated, but Infinity War proved it's actually always been really simple too. "The other guy" cares about his other guy more, and when he realized he wasn't strong enough to keep him safe he got scared. Hulk smash? No, Hulk scared—scared he no protect Bruce.
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