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In Avengers: Endgame, we are told that 'somehow' Bruce experimented on himself and was able to merge his Bruce and Hulk personas to become 'Smart Hulk.'
Is it ever mentioned how he was able to do that?
How did he go from not being able to become the Hulk in Infinity War to becoming Professor Hulk in Endgame?
In She-Hulk: Attorney at Law S01E01 "A Normal Amount of Rage," Bruce reveals to Jennifer Walters (She-Hulk) that during the five-year period between Avengers: Infinity War and Avengers: Endgame, he worked on integrating his two personalities at a lab in Mexico built by Tony Stark.
Related dialogue:
Bruce: This is where I spent The Blip fixing myself and integrating the Hulk-Banner identities.
Jennifer: In this tiny beach house lab.
Bruce: [Chuckles] Yeah. Tony built it for me a few years ago.
Bruce: Jen, I have been struggling with this for years. The best that I could do was merge my two halves. Give it time in the lab, we could possibly do the same for you.
One of the techniques Bruce teaches (or rather, tries to teach) Jennifer on "how to Hulk" is dialectical behavioral therapy, which "allows [a person] to hold two opposing truths at the same time." This most likely played a big part in how Bruce was able to merge his two personalities into "Smart Hulk," or "Smug Hulk," as Jennifer prefers to call him.
Jennifer: Teach me how to Hulk, please.
Bruce: Dialectical behavioral therapy. It allows us to hold two opposing truths at the same time. First, we're gonna practice some mindfulness with a few yoga techniques.
In Endgame Bruce says he spent "18 months in a Gamma lab" to merge Banner's personality with Hulk's body. In She-Hulk he tells Jen about "merg[ing] my two halves".
As we see, it seems the long-time gamma exposure kept Bruce permanently in Hulk form, having to use the device in She-Hulk to keep himself in a human form. He either through meditation (as we see him doing in She-Hulk) or his studies was able to suppress the Hulk's personality and let Banner stay in control of both forms.
My personal theory:
Hulk allowed Banner to take control on purpose. I think he's chilling in the back of Bruce's mind waiting for a chance to come out again. I don't think we've seen the last of the Hulk.
