If you ask any group of people what the basic staples of life are they will give a short list that goes something like this: air, water, food, and shelter. The very basics.
Some of the people you ask will then go on to personalise their lists, citing their "need" for, often, much less basic things, like sex, or the internet, or one of millions of things they can't imagine living without, but could, if they had to.
The one absolute necessity that people almost never think to put on their lists is basic human touch.
It's almost odd how little it's taken into consideration until it's not there any more, until there's no one there any more, and suddenly it's just constantly on the edge of your awareness, a sharp wrongness beneath your skin. It leaves you twitchy, on edge and tipping; it leads you further away than ever from humanity, locked inside your own skin.
During the dark, early years in his struggle for control, Bruce managed to keep from touching another person for two years. He would have secluded himself forever in his own custom hell, unescapable, even with a bullet, if it weren't for an accident. (Fitting, considering how he got into this mess in the first place.)
He was in a village, small, even for Laos, trading. He brought fish he had no way to preserve to trade for some rice and the dried version of a local pepper. Everything was fine. He was under control. He knew enough of the local dialect to cobble together a conversation with a woman in the village who knew maybe thirty words of english and a bit of Spanish, for some reason. He didn't care to haggle, but that's how trade was done in that part of the world and they were all friendly enough that he argued. It was actually a pretty great day.
Then, as she handed him the rice, their fingers brushed. It was so small and insignificant and lasted less than a second, but it was so utterly human and suddenly he ached.
She didn't even look startled when his eyes flashed green, just curious. They looked worried when he stopped bargaining and simply agreed, obviously frayed and desperate to leave, but they let him go without comment.
Bruce collected his things and disappeared back into the forest, breathing deep and slow. For a long time each breath was the the whole of his existence, warm and sweet in his lungs.
When he was calm again he resolved to never do anything like that ever again. He needed to control himself better, to seal himself off further from the world.
The second time he touched someone he wondered if he did it on purpose. Well, not really on purpose, not consciously, but maybe just as a need presenting itself. That was... not good. He couldn't do that. What if the Other Guy just came out? It's not like they could fight him, even the most powerful army on Earth took him on and lost on a fairly regular basis before he ran. People were going to get hurt if he couldn't at least control his unconscious.
It didn't occur to him until hours later that he'd been so wrapped up in his motivations and the consequences of his actions that he didn't even notice when he didn't turn the slightest bit green. Rusty, startled laughter floated through the conopy.
By the time Natasha found him he trusted himself enough to be an occasional doctor. He still touched his patients as little as possible and avoided all other forms of contact all together, but it was close to enough.
He almost felt guilty in how much comfort he took from looking after his patients. His past could not be atoned for and he didn't deserve it, but that wasn't their fault and he could help them. That would just have to be close enough, too.
It was probably inevitable that Tony was the first to touch him casually after what felt like centuries of self-imposed isolation.
"Self-destructive" would turn out to be a key phrase in Tony's file, he would find out much later, during a rare peaceful moment in a quiet office, but he didn't think that was what it was when Tony reched out to poke him playfully in a tiny room full of breakable things. It felt teasing and childish and so utterly human that he had to catch his breath for a moment. And Tony just kept talking about science.
After Earth was saved the other Avengers began to feel comfortable enough to touch Bruce from time to time. Steve would pat him on the back after successful missions, or rest it there in comfort when things went less than well. Thor began to include him in the bone crushing hugs he pulled whatever person nearest to him into whenever he finished Skyping with Jane. Clint was just as likely to drape himself on Bruce's back when he didn't feel like standing up on his own in the mornings as he was to anyone else but Natasha, whe was too grumpy to deal with him before a cup of coffee. Even Natasha came around with the occasional nudge or companionable press of shoulders during transport. But Tony perhaps touched him the most often, and seemingly without thought. He'd poke and prod and push and pull and never once did he doubt Bruce's control.
For all that, Bruce was never the one to initiate contact, not with anyone.
Pepper left Tony two years after the Avengers formed.
Bruce found him in his workshop, drunk and drinking, hugging Dummy somewhat desperately with the hand that wasn't holding a $500 bottle of scotch. He sat on a nearby workbench and listened as Tony rambled incoherently about not being able to give up everything that made him himself, even for Pepper, even if he sometimes wished he could. When he finally broke down and began to cry, Bruce reached out and pulled him into a hug. They stayed like that for a long time.
The first few months were almost an exact reversal of their previous relationship; it was Bruce's turn to poke and prod and push and pull, to get excited over crazy experiments and crazier adventures.
The other Avengers respected his wishes and let him grieve alone, but if there was one thing Bruce knew about Tony Stark it was this: the more he pushes people away, the less he wants to be left alone.
Another defining character trait Tony had was one Bruce was mostly unaware of: he had the unfortunate habit of mistaking kindness for love.
Bruce found this particular piece of information out one night as they were studying some nanobots of unknown origin, heads bent close over a microscope. He was explaining his theory that they infected their hosts like a virus when a warm, callused hand with blunt fingers curled around the back of his neck and drew him away from his observations and into a kiss. Unsurprisingly for a man who kissed so often, he was good at it. What was a surprise was how chaste it was. It was a warm, dry press of lips and a hand holding him in place and nothing else. For what felt like the longest time Bruce just stayed there, too stunned to even register anything beyond the sense of touch.
Then his brain kicked in and he scrambled back, making it to the elevator just in time to trap the Other Guy inside, if only for a moment.
The next morning no one asked what was with all the property damage. Natasha gave him a cup of tea and told him Tony had been called to Tokyo on urgent buisness the night before. He couldn't tell how much she actually knew and she didn't tell him.
It wasn't that he didn't like Tony, nor was it a gay freak-out (even if it was a bit of a surprise to discover that he might've actually enjoyed it). No, it was just that Tony was his closest friend and he was so scared that even if the Other Guy didn't exist he'd find so many ways to mess it up. He ignored the voice in the back of his head telling him that he already did.
Better to stay friends.
Evidently, Tony agreed. He showed up a few days later a bit hungover, but no worse for wear, acting like nothing had happened.
So, for all intents and purposes, nothing did.