Between a spectacularly skittish mother and one hell of a drunkard for a father, Bruce Banner had much to be desired in the department of healthy relationships growing up. He was a social man at heart—the negative attention he received at home translated to an intense desire to act out in school, to become the lovable class clown. Any attention at all was better than what he was getting: infrequent (albeit loving) words from his mother, books thrown at him by his father, and the occasional lethargic stare from the useless old lump of fur that was their cat. It was only the promise of bottles breaking across his face that kept Bruce from pulling prank after prank and, instead, focusing energy on his studies.
His mother's death hurt more than he'd ever let on. She was the only person he'd ever had the chance to be close with, and what had his monster of a father done? He'd killed her. He'd have killed Bruce, as well, if the boy hadn't fought back so damn hard. He was rather rapidly shipped off to live with his aunt, who was great. At least, Bruce thought in hindsight that she was probably great. After the traumatising clusterfuck that was his childhood, it seemed foolish to invest any energy in becoming close to a woman that he'd probably only know for the year it would take him to finish high school.
By the time he started studying nuclear physics at Desert State University, Bruce felt like he'd explode with all of the things he'd bottled up over the years. Professor Weller was the only person Bruce talked to on a regular basis, but they were by no means close. The first person Bruce could call a friend in his twenty-plus years was Walter Langkowski, a classmate at Penn State. It was slow going, and horrendously awkward for the first month or so, but in the semester that they worked together, Walt managed to teach Bruce what it was like to be properly social.
It wasn't until Betty that Bruce really got the chance to put what he had learned into practice, with coffee and dinner and the whole nine yards. It was nice, having her around. It was better than nice, actually, it was—well. If Bruce had been at all a romantic (which he most certainly was not, no matter what Betty would say), he would have called it perfect. There was no other word for it, really. After so many years of loneliness, having even just one person he could share everything with was… intense, to say the least, in the exhilarating way that he had always expected love to be.
But then there was the accident, and Bruce went from a metaphorical timebomb of emotional damage to a literal timebomb of rage and oh, god, there was no way he could be around anyone, not when he'd fly into a murderous rampage at the slightest provocation.
So he ran. He hid himself away in the Peruvian Amazon, as far from civilisation as he could manage. He hid himself in the outskirts of Sao Paulo, struggling for social contact while the rest of his rational mind screamed at him to find the Marianas fucking Trench and drown himself there, because there was the possibility that he could destroy the whole city through half a heartbeat's worth of anger. Months later, it was tiny towns in northern Brazil, where he'd rent a room and get a job if only to maintain a semblance of humanity.
After Harlem, he had stowed away on the first ship to cross the Atlantic, choking back dual urges to throw himself into the most populated cities in the world and to keep himself away from every living organism in a two mile radius. His desire for company won out, in the end, and he slowly started sinking back into the world. Brief experiences with sick patients were as much as he'd allow himself, even after months and months of training. He hadn't lost his cool in over a year, but the monster was still there in the back of his head, waiting for lock on his cage to break and allow him to come ripping out.
He hadn't realised how bad it was until SHIELD dragged him back to the States—until he'd been thrust into a situation where he was on a team, and mutual trust was vital, and it had been so easy to just relax for the time being and let himself enjoy Tony's constant company, Steve's check-ins, Thor's food runs… Of course, it didn't last. It couldn't last, what with Bruce's condition. Save Tony (who really didn't count, if you took into consideration all of Tony's self-destructive habits), he was certain that everyone he came into contact with would become terrified of him—the Other Guy, not him, the Other Guy—soon enough.
No. Stop it. That thinking was ridiculous. Bruce needed a friend, damn it, and there were four (five? He still wasn't sure where he stood with Thor, seeing as the Asgardian left almost as soon as the fighting was over) people in their team who seemed perfectly willing to fill that position. He couldn't spend the rest of his life being too invested in what-ifs to form emotional attachments—not when he had a gaping hole in his chest that normal people filled with family and friends. Family, he knew, wouldn't be happening. At the very least, he could manage friends. At this rate, he'd probably be driven to insanity if he didn't find someone to keep him company soon.