WARNING: The following contains major spoilers for Hulk #1, on sale now from Marvel Comics.
The Hulk is one of the five original Avengers, but he has always had a difficult time fitting in with the rest of the team. In fact, quite a few classic stories involve Earth's Mightiest Heroes banding together to fight the gamma-powered giant, often treating him as more of a threat than a friend and teammate.
Considering that the Hulk's power is channeling rage into strength, it should not surprise anyone that he has turned friends into enemies on multiple occasions. In fact, he is even at war with himself, and that internal divide is at the root of how he has managed to turn two of the founding Avengers into his greatest enemies. It is also why he will never find peace.
It should be noted that the Hulk and Banner are two separate individuals and one of the Hulk's greatest enemies is Banner. The resentment runs both ways. This takes on new dimensions in the light of a revelation that the Hulk might not be a psychological manifestation of Banner's childhood trauma, as was originally suspected. Rather, Doctor Strange suspects that the Jade Giant manifested specifically to protect the world from Bruce Banner's destructive rage, as revealed in Hulk #1 (by Donny Cates, Ryan Ottley, Frank Martin, and VC's Cory Petit).
The rivalry between these two personas stretches back to the beginning of their existence together. Not only do they fight for control of the same body, but they resent each other on a personal level. Hulk sees Banner as a puny scared weakling, while Banner believes that the Hulk ruined any chance he had of ever enjoying a normal life.
The other rivalry of note is between Hulk and Tony Stark (AKA Iron Man), and this is somewhat more complicated, as both Banner and Hulk have issues with the armor-clad Avenger. Stark's arrogant showboating and constant belittling of Banner is a pretty egregious example of unnecessary bullying. This behavior has also negatively impacted Banner's work as a scientist. Meanwhile, Stark's tendency to think he always knows best often comes at a cost to everyone around him and has made him into one of the Hulk's greatest enemies as well. Stark, alongside Marvel's Illuminati once tried to banish the Jade Giant from Earth by sending him rocketing to another planet against his will. On top of that, the very fact that Stark has built multiple versions of a "Hulkbuster" armor is illustrative of just how far the billionaire industrialist has gone to cause Hulk pain.
The Hulk is one of the strongest beings in the Marvel Universe, but if he was created just to stop Banner, that suggests Bruce Banner is potentially one of Marvel's greatest villains. Banner just seized control of the Hulk's body, and Tony Stark made the painful mistake of provoking him once again, proving himself to once again be one of the greatest enemies of both personas.