Bruce Banner never wanted the Incredible Hulk to live rent-free in his body and has tried to kick him out for good multiple times. Hulk doesn’t want to be Banner’s roommate either but every time either one moves out they always come back together. Like a pair of catty roommates that can’t ever get themselves to split because they both love the drama, they both secretly like being together.
Banner became bonded to the Hulk after being exposed to gamma radiation following an accidental explosion that should have killed him, but instead, gave him a new shadow to his every step. Hulk takes over when Banner loses emotional control, typically due to bouts of rage, and goes on a destructive rampage. Over the years the two have sometimes cooperated, often been at odds, and almost never been friends. Hulk lives as a separate entity inside Banner’s consciousness and has several times been split from his human host.
Usually Banner is the one that initiates the separation. It’s always a messy divorce but Banner has had times of total control over the simple-minded Hulk. In Incredible Hulk #130 by Roy Thomas and Herb Trimpe, one of Banner’s old classmates develops a Gammatron intending to erase Hulk but just divides the big green rage monster from Banner so that Hulk could try and kill his former landlord. But both realized they are tandem spirits and if one was killed they both would die. Other heroes have tried to cure Banner including Silver Surfer who siphoned off the gamma radiation to rid Banner of Hulk. But something always ends up needing smashing so Banner comes crawling back and merges once again with Hulk.
Hulk has tried to rid himself of Banner and with some success. Hulk had Doctor Doom perform brain surgery and remove the gray matter that was Banner’s consciousness in Incredible Hulk #7.1 by Jason Aaron and Jefte Palo. Without the puny Banner along for the ride Hulk roamed the world, but even swimming with whales, spit-roasting sharks, and soaking in a hot tub got lonely and he wanted Banner back - which was handy because Banner had reintegrated with Hulk through some unexplained means already and was just dwelling in Hulk’s subconscious.
It’s been theorized that Hulk embodies the traumatic emotions that Banner has repressed and turned into his own hated monster. Projecting all his vile feelings on an unstoppable rage monster suggests Banner went through some traumatic stuff. Banner had a rough childhood thanks to his alcoholic and abusive father, Brian Banner. The Hulk could be a manifestation of Banner’s negative emotions that were repressed as a result of the abuse he suffered. Banner actually killed his own father, accidentally knocking his head into the headstone atop his mother’s grave. Banner repressed the incident and convinced himself the murder didn’t happen, persuading himself that the two fought over his mother’s grave but that his father was later killed by muggers.
Hulk and Banner’s story is a helpful guide to confronting negative emotions. Hulk and Banner are inseparable because they are two parts of Banner’s emotional spectrum. Even though Banner loathes Hulk and Hulk has no love for Banner, they need each other. Banner is the Ego to Hulk’s Id and they each need the other to be a balanced person. Even though they are distinct personalities with unique perspectives and thoughts, they are still two halves to a whole.