Avengers: Endgame introduced a new version of Bruce Banner when he merged himself with the Hulk by spending eighteen months in the gamma lab to “put the brains and the brawn together.” It was his solution to years of having to manage his Hulk-rage as he understood that it was a result of him treating the big guy like a disease. However, in a since-deleted post, an eagle-eyed viewer on Reddit has put forward a theory that the Hulk-Banner merger began long before the events of Endgame.
In the theory, the user stressed the fact that Banner never retained complete memories of his time as Hulk. He was always aware that he brought down destruction and death in his beast form but failed to remember the specifics, which varied depending on whether he transformed under stress or voluntarily. But in Avengers: Infinity War, after Heimdall beamed Hulk away to Earth, Bruce told Tony that Thanos is coming after he killed Thor. He even explained that the Mad Titan has a habit of annihilating half the population of planets and has acquired two of the Infinity Stones. Somehow, Bruce remembered things that happened when he was the Hulk.
This incident hints at the possibility that while Bruce embraced Hulk a lot later, his alter-ego had been trying to merge with him a long time before that. And when looking at it through that lens, there have been plenty of signs in the Marvel Cinematic Universe that the Hulk was evolving beyond his mindless persona.
In The Avengers, Hulk saved Tony after he fell unconscious and was falling through the sky. When Tony didn’t wake up, the Hulk, even in his rage, wasn’t ready to accept that he might be gone and roared in denial, which coincidently awoke Iron Man. Up until then, the Hulk largely acted on his instinct of self-preservation and was ready to attack anything that he perceived as a threat. This was the reason that Banner always found it difficult to keep the green guy subdued if he was stressed or angry.
Then, in the Avengers: Age Of Ultron, Banner demonstrated exemplary control over changing into Hulk with the help of Natasha Romanoff. But he was evolving differently than his alter ego was -- despite almost mastering the ability to transform at will, he still had a hard time recalling the events that happened when he was Hulk. Hulk, on the other hand, became increasingly aware of Banner and visibly started exhibiting signs that his psyche was merging with Banner's He connected with Banner’s feelings for Romanoff, going on to save her at the end of the movie. Another sign of his heightened awareness is that he could now stall Banner from transforming back -- by the time Thor meets up with him in Thor Ragnarok he had been in his Hulk form for two years.
During his time on the planet Sakaar, Hulk didn’t just learn to string together two words to have a conversation and calm his all-time destructive rage, he also got in touch with Banner’s other feelings. Banner always thought that Earth doesn’t want a monster like Hulk and went to great lengths to keep him under control. When Thor, unable to convince Hulk to help him escape Sakaar, criticized him for how Earth hates him, he is visibly affected and hurt. But he reacted more like a tantrum-throwing child, and in many ways, he was one -- just getting the hang of feeling emotions different from red rage.
So, By the time Thanos attacked, destroyed half the Asgardians and seemingly killed Thor, the Hulk was more Bruce Banner than he was aware of. The rage-filled beast, who only had one frequency, tried to save his friends from imminent doom. Later in the film, Hulk even consciously stopped Banner from transforming as a display of his childish sulking over the continued attempts to control him.
When Banner finally merged himself with Hulk, he explained that the snap made him see his alter ego from a new perspective. It's possible that it was him finally understanding that Hulk had been trying to merge with him all this time that made him finally embrace his other personality.