Whether you loved or hated Avengers: Infinity War, if you're a fan of the Incredible Hulk then it's tough to not be disappointed by his absence from most of the movie. The Hulk withstands a brutal beating by Thanos before being whisked away to Earth by Heimdall, and that's the last time you see him in his full glory. Bruce Banner tries and fails several times to get his greener half to come out and play, but the Hulk refuses to emerge. At most, we get a few brief shots of the Hulk's face replacing Banner's long enough to yell “NO!” And then he's gone.
So why is the Hulk shy all of a sudden? Why won't he help with the fight against Thanos? There are some obvious answers, most of them leading to the trauma of what he endured at the hands of Thanos. But there are some less obvious — and possibly much more likely — answers that fit more with what we know about the Hulk.
We have a Hulk and he's scared
Fear isn't an emotion usually associated with the Hulk, unless you're talking about how other characters feel about him. So the idea that the Hulk is refusing to emerge because he's afraid of Thanos may seem tough to swallow. After all, at the end of Thor: Ragnarok, the Hulk was willing to go toe-to-toe with a Godzilla-sized Surtur, even after the demon flicked him away like an annoying flea. Hulk seemed genuinely annoyed that Thor didn't want him to take another crack at the beast.
But it's still possible what's stopping the Hulk is fear, and maybe not even his own.
The beating the Hulk suffered from Thanos is unlike anything he's endured before in the movies. The only other fight the MCU Hulk has lost is his brawl with Iron Man in the Hulkbuster suit in Avengers: Age of Ultron, but he never saw the final punch coming and never had the chance to think about what was happening to him, not to mention that it was a fight he didn't really want in the first place. Scarlet Witch forced the Hulk to emerge and go on a rampage. It's possible he doesn't even remember it. The Hulk's intelligence and awareness, independent of Banner, has grown since his fight with “Veronica.” He knew what was happening to him as Thanos beat him. It's something he's never experienced before, and he may not know how to handle it.
Father of mine
One regrettable aspect of Bruce Banner's portrayal in the MCU is that — as popular as Banner's “I'm always angry” line from Avengers has become — the movies have never explored exactly why that rage is always with him. It's possible that directors Joe and Anthony Russo are using Thanos to finally give us a glimpse of that story.
In the 1985 comic Incredible Hulk #312, longtime Hulk writer Bill Mantlo wrote one of Bruce Banner's most memorable and influential stories. Most of the tale is a flashback to Banner's younger days. We learn that both Bruce and his mother Rebecca endured extreme emotional and physical abuse at the hands of Bruce's father. The father eventually murders his mother for trying to escape from him. Mantlo essentially told readers that the gamma bomb blast from 1962's Incredible Hulk #1 didn't create the Hulk. The trauma Banner endured created the Hulk — the gamma blast just set him free.
It's possible that what's stopping the Hulk from emerging in Infinity War is not that the beating he took from Thanos scared him, but that it reawakened the childhood trauma.
In fact, this could be why a certain bizarre scene was deleted from Thor: Ragnarok. In the scene, Banner expresses regret at missing his father's death because he was too busy working. In the case of the comics, that would seem strange considering not only did the comic book Banner have no fluffy feelings for his father, but it was eventually revealed that Bruce Banner actually killed his father. If the Infinity War filmmakers are planning on using the Hulk's refusal to emerge to introduce the abuse backstory, the conversation from the deleted scene wouldn't make sense.
A Hulk humiliated
Maybe the Hulk isn't scared and maybe the Hulk isn't traumatized. Maybe he's just embarrassed.
As was mentioned earlier, the Hulk has never experienced the kind of utter defeat Thanos handed him in the opening of Avengers: Infinity War. And the Green Goliath doesn't just have the intelligence of a toddler; he has the emotional maturity of one too.
The beating he took may have destroyed his confidence. The Hulk can't fix a Helicarrier engine like Iron Man or plug an interstellar portal with lightning like Thor. The Hulk has one job: beat the tar out of whoever needs a beating. If he can't do that then, in his mind, how could he be anything but worthless?
It's worth remembering that not only is Banner around other Avengers each time he tries to change into the Hulk in Infinity War, but there's a specific Avenger he's being reminded of — Tony Stark. And who is Tony Stark to the Hulk? Well, among other things, he's the only guy other than Thanos who's managed to knock the Hulk out. The first time Banner tries to change in Infinity War, Tony's right there, urging him to Hulk out. The second time, during the battle in Wakanda, Tony's nowhere to be found but Banner is inside the Hulkbuster armor. He's inside the suit that not only was used to give the Hulk one of his only defeats, it was created solely for the purpose of beating up the Hulk. It's possible that it was the wrong time to remind the Green Goliath of that.
The Hulk blames himself for Thanos
It's possible Hulk's reasons for refusing to fight are less about fear and humiliation, and more about guilt.
Right after the Hulk is defeated by Thanos, Heimdall uses his last few breaths to transport Hulk to Earth so he can warn them of Thanos. That means the first time Banner tries to Hulk out, as far as the Hulk knows, he was the only survivor of the Asgardian refugee ship. As far as he knows, Thor, Valkyrie (a.k.a. “Angry Girl” in Hulkspeak), and everyone else is dead. He probably couldn't care less about Loki, but Thor and Hulk formed a bond during Ragnarok and thinking he wasn't strong enough to save Thor from Thanos would be crushing to the Hulk.
It may be that a number of toy and collectible releases support the notion that it was guilt holding the Hulk back: specifically toys and collectibles that depict Hulk bursting out of the Hulkbuster suit. If it was originally planned that Banner would finally succeed in transforming into the Hulk while in the Hulkbuster suit — since we know it's in Wakanda where Banner is in the suit and it's in Wakanda that Banner eventually learns that Thor is still alive — it could be that the plan was that the revelation that Thor didn't die is would be what helped the Hulk defeat his guilt.
Third rock from the sun
The reason Hulk refuses to emerge may not have anything at all to do with Thanos. Ironically, it may be that he's too damn angry.
Thor, Loki, Valkyrie, and even Banner may have been fine with leaving Sakaar in Thor: Ragnarok, but the Hulk didn't have much say in the matter. During their first one-on-one dialogue scene together, Hulk tells Thor plainly he has no intention of returning to Earth. “Earth hate Hulk,” he tells the thunder god. Thor and the Avengers? They're “Banner's friends.” Not his.
On Sakaar, the Hulk was treated better than he'd ever been. Not only was he not treated like some horrifying extension of Banner, but no one even knew about Banner. The people of Sakaar didn't run screaming from him for smashing his enemies; they cheered him. They had parades for him. They built a huge Hulk face on the planet's tallest building and gave him is own Hulk-friendly hot tub. Why would he want to leave?
Of course he knew he wasn't on Sakaar during the battle in Asgard and later during the journey on the refugee ship, but with his thick head and slow mind, the Hulk may not have truly grasped that he was going right back to the last place he ever wanted to be. Back to Earth where he was hounded, where he had no friends, and in fact where he was likely still wanted for the death and destruction he caused during the Johannesburg rampage of Avengers: Age of Ultron. Once he was in Greenwich Village, with Tony Stark and other puny humans surrounding him, maybe the Hulk was angry that he was right back were he started. Maybe he didn't want to fight to protect the last place he wanted to be.
Hulk thought Hulk was done, and then they pull Hulk back in
There's another reason why the Hulk's anger might be what's preventing his emergence, but it has less to do with what planet he's on than whose head he's in.
In Ragnarok Bruce Banner was convinced that if he changed into the Hulk, he might never change back. He tells Thor this shortly after Hulk's transformation to Banner in the Quinjet. We learn the Hulk had been in control since Black Widow forced Banner to change into the Hulk at the end of Age of Ultron's conclusion and that, while before he always felt he had “one hand on the wheel” when he was the Hulk, during these past two years he felt that the Hulk had taken the keys and locked him “in the trunk.”
What happens seems to confirm Banner's fears. After changing into the Hulk in Asgard, we never see the Hulk change back into Banner until Heimdall whisks him away to Doctor Strange's Sanctum Sanctorum in Avengers: Infinity War. As far as we know, from end of Thor: Ragnarok to the moment he's sent back to Earth, he's green the whole time.
During the two years between Age of Ultron and Ragnarok, the Hulk managed to stop himself from changing back to “Puny Banner” even once. The Hulk may have been convinced that he was finally free of his weaker half — free of losing control to Banner, free of being used like a tool by his fellow Avengers and allowed out only when they deemed it necessary. During those two Banner-less years, the Hulk got to feel in control. Maybe, like Banner, the Hulk thought once he came out in Asgard that would be the end of it and he'd be in charge for good. Now that it's clear that's not the case, maybe the Hulk doesn't feel like being Banner's tool anymore. Maybe if he isn't the only one who gets to play the game, then the game isn't worth playing.