Avengers: Endgame showed audiences a Bruce Banner that had successfully merged his persona with the Hulk. "Smart Hulk," as he has been dubbed by fans and filmmakers alike, combines the brains of Banner and the brawn of Hulk, and the path to unify the two was one littered with failure and fear.
The beating the Hulk took from Thanos at the beginning of Avengers: Infinity War is enough to force him to retreat inside Banner. He refuses to come out to help, which, according to the filmmakers, is because Hulk is tired of being used to solve Banner's problems even when the fate of the universe is at stake.
This ultimately results in Banner's failure to stop Thanos during the battle in Wakanda, but it is more than the Hulk's protest that holds him back. It is also fear. While this character turn has been hinted at in previous films, it is most accurately explained over 20 years ago in the pages of Superman Vol. 2 #175.
On the anniversary of Superman's death at the hand's of Doomsday, the two battle again. Doomsday, who was previously a mindless monster, has evolved in intelligence and emotion, which is similar to Hulk's evolution from savage beast to "Smart Hulk." Although Doomsday is more intelligent, Superman doesn't see this as an advantage. Instead, he takes the opportunity to lecture Doomsday, and the words he uses draw unintentional parallels to the Hulk's character arc in the Marvel Cinematic Universe.
"You're different now. You can think for yourself," Superman tells Doomsday. "So think about this. Before, you were a mindless thing. Nothing could hurt you. You couldn't feel pain. Much less understand it. But, once you have felt it -- it changes you forever. And you'll begin to understand something new. Fear."
The reason Hulk refuses to emerge in Infinity War has been discussed by both fans and the filmmakers. A popular theory is that Hulk held out due to his fear of Thanos. This is refuted by the previously referenced explanation from the filmmakers, and Hulk's actor, Mark Ruffalo, adds that, "the only thing Hulk is afraid of is Banner." However, when both explanations are interpreted with the Superman quote in mind, fear remains the operative motivation.
Banner is used to experiencing fear. He spent years fearing the Hulk, only using him to deal with his most dangerous problems, like the Chitauri invasion of New York or Ultron's attack on Sokovia. Hulk, on the other hand, only knows anger and power. Through all of his battles with the Avengers, the Hulk never had to experience the same fear that Banner lives with daily. That is until he faces Thanos.
It isn't Thanos that Hulk fears, specifically, but rather fear itself. Fear of pain, fear of failure, fear of being used as a tool and fear of lacking his own identity. As Superman expresses to Doomsday, "once you have felt [pain] -- it changes you forever."
In Infinity War, Hulk is coming off of a series of painful losses. Like Doomsday, he is no longer an uncontrollable rage-monster, but rather he is a thinking being with a complex array of emotions. The Hulk in Infinity War has spent two years being himself, cultivating his own personality and winning countless battles in the Colosseum on Sakaar. This is not the same Hulk that audiences see in MCU films prior to Thor: Ragnarok.
After Banner fails to stop Thanos from wiping out half of all life in the universe, he and the Hulk are finally at a point where they both need each other. As Banner, now "Smart Hulk," explains in Endgame, "I started looking at [Hulk] as the cure." Rather than viewing the Hulk as some kind of disease to be rid of or as a weapon to be used, Banner accepts the Hulk as a part of him. During 18 months in a gamma lab, Banner is able to merge the brains with the brawn.
However, "Smart Hulk," may not be Banner at his fullest potential. Returning to Superman's quote, fear is what caused his change. Banner may claim to have found peace between both sides of himself, but with fear as the motivation there could be serious consequences.
As much as "Smart Hulk" is the best of both Banner and Hulk, he represents both of their fears, with Banner hiding behind the brawn and Hulk hiding behind the brain. If this chain of fear ever breaks, the MCU could see a Hulk that isn't held back by either side of himself. Considering the tremendous pain "Smart Hulk" endures by using the infinity stones in Endgame, he may be due for another drastic and possibly dangerous evolution.