Bruce Banner was a cautious man and it wasn't just because he turned into a radioactive rage monster or that the army was trying to kill him; no, it was something more personal. Even when he was in total control of the other guy, isolated in the middle of nowhere, he still needed his fix. In his mind, he described it as an itch.
In essence, it was a compulsion to care for and aid injured people. As a doctor it helped him feel connected to and appreciated by his patients, which in turn made it easier to diagnose and treat issues other physicians would have overlooked. The problems started if there was no one to help or heal. The itch would transform from a beneficial tool into something very dangerous. Instead of feeling needed and respected by the people around him, he felt trapped and desperate, so he would start to plan. Then suddenly people would start to have more and more "accidents". Accidents that Bruce always happened to be there for and more than willing to help with. Bruce knew he wasn't safe to be around. There was something wrong with his head; he couldn't stand to see people hurt or sick, but he always had to have patients to take care of. So much so, that if he couldn't find someone bleeding or broken, he'd make one instead. People always got hurt around him. Either the Hulk wrecked a town or, when the itch became too much to handle, bad things would start happening to nice people.
Luckily, Bruce had found a fairly simple way to keep it in check. While hiding in multiple third world countries, Bruce found that a lot of people needed a doctor, but couldn't afford a hospital visit. So Bruce would make house calls any time, day or night. In fact, he had such a well-established network of poor and downtrodden people in India that the itch was never more than a barely-there prickle. He didn't do it for the little money that they could give him, because really there was no way he could have lived off of that. No, he did it to make sure the itch stayed useful, instead of destructive. Then he met Natasha Romanoff.
He hated that SHIELD had dragged him out of his comfortably obscure living arrangement in Calcutta. He hated that nobody on the whole helicarrier trusted him, he hated that he was put in a war zone that made him Hulk out, but most of all he hated that he couldn't leave the team to fend for itself. He should have run away and gone back to his safe place with its constant stream of people in need, but he couldn't. Maybe it was because he had been alone for so long or maybe he was just tired, but when Tony Stark asked him to stay he said yes.
Tony believed in him and appreciated his mind. Tony wanted Bruce to come play in his freak'n awesome lab. Tony wasn't scared of Bruce even though he'd seen what the Hulk was capable of, so he stayed. Bruce lost the battle to keep himself distanced from the limelight, but he promised himself this new arrangement would only last as long as the itch stayed at bay. Luckily for Bruce, Tony had been so beaten up after the battle with the Chitauri and then falling from the sky, after almost dying in space, that the itch wouldn't be a problem for a long time. Of course Stark had the best doctors that money could buy, but Bruce was the only one willing to live with him in the Stark Tower labs until the penthouse was fixed. Tony also had some rather self-destructive habits. Pretty much a live-in doctor was needed to keep an eye on him so he wouldn't reinjure himself by testing out a new invention or drinking himself into a coma before he was properly healed.
The first month and a half of living in the tower was great. The labs were mind-blowing and Stark's company and ideas keep him nicely busy. Tony's multiple injuries kept the itch down to a whisper and things were almost relaxing. He got to know the team better, under much less stressful circumstances and they seemed to like him for his demeanor, brains and the Hulk's brawn too. The difficulty didn't start until Tony pronounced himself completely healed and told Bruce that he could now stop "mothering" him.
The itch started small, an annoyance that wouldn't go away when he tried to meditate or sleep but after a week, of no one even getting a scratch, it amplified. He should have just left right then, packed his bag and headed back to his network in India, but Tony had looked so upset when he mentioned leaving, god damn those kicked puppy eyes, and now everyone on the team wanted him to stay. They were all trying really hard to make him feel welcome, he missed being around people who actually cared, but if he was going to stay, the itch couldn't. It was constant and unbearable inside his brain, he wished he could take off the top of his head and just rub it away but he knew there was only way to stop it. He needed his fix. Bruce felt absolutely horrible about starting to plan, but if he started out small, just enough to channel his doctoring instincts, maybe things would work out for the best. Something that was easily treatable, but that took time and attention to heal.