Tony was everything Steve wasn't.
He was brash and rude, uncaring and highly judgmental. He pushed people away, not necessarily because he was afraid of hurting them, but because he felt he didn't deserve them. Then, in the spare time he had that would've gone towards social interaction, he dedicated himself to building suit after suit after suit, all in an attempt to make himself into someone likeable. Because, really, for as confident as Tony was, that was all the billionaire wanted. To be liked. To stop being looked at as that kid in his father's shadow, that kid who got everything he had in life thanks to his dear, much better than he would ever be, dad.
He didn't easily or willingly show his emotions. Most days, the only way Steve could tell what the guy was thinking was by staring him straight in the eye; if there was one thing the genius hadn't mastered, it was hiding the expression show by those coffee-brown eyes. They were like a clear picture into everything he was thinking about; just one look could tell you if the man was actually listening to you or if he was busily crunching numbers in his head, it could tell you if that cocky smirk was genuine or if you had struck a deep nerve. Steve loved how emotional and expressive those eyes were, his hands itched to try and draw them, although he knew he'd never be able to capture them just right. Of course, those eyes would also tell him how long it had been since Tony was healthy, since he'd had a goodnights sleep or last felt contently full.
Because Tony didn't take care of himself, often spending days at a time holed up in his lab with no outside contact aside from whatever news Jarvis felt was important enough to alert him of. He'd finally finish whatever changes he was working on, test it out and see if it would work. If it didn't, he'd grow panicky and stressed and he'd stay in the lab even longer, often times with an added companion; alcohol. If he did, he'd be all smiles for a day or so, going upstairs to socialize and eat and for a moment managing to put Steve's worries at rest. Then he'd think of something else he could do, or he'd wonder if he had made the right choice in these modifications, or he'd think that this just wasn't enough to make up for all the bad things he blamed himself for. And then he'd race back to his lab and seal himself off again, and the whole process would repeat. And every single time, it nearly broke Steve's heart.
Tony wasn't a bad person. Deep down, Steve knew that, and he was pretty damn sure the rest of the Avengers did too. After all, he didn't have to take them all in; S.H.I.E.L.D had already offered most of them small apartments in the city. Tony had just refused to let them take any of it, going as far as actually intercepting Fury's attempts to move them out (them mostly being Steve, because if there was one thing Fury seemed worried about it was Iron Man fucking up Captain America with fake history lessons or just generally getting onto the Captain's nerves until he eventually broke and beat the crap out of him). When Steve had mentioned he was considering trying to find a job, Tony had even offered to put him on Stark Industries payroll so he'd have pocket money (Steve refused, of course, and the prospect of a job was never mentioned again out of fear that Tony would actually make good on his offer. Steve was already uncomfortable living in the extravagant tower without rent; he didn't need Tony throwing money at him too).
Steve blamed most of Tony's insecurities on his childhood. As much as he looked up to Howard Stark, even he had to admit he had definitely screwed up Tony's childhood. In a way, Steve blamed himself for that. It was just one more unforeseen consequence the serum had; if he had never taken it, Howard wouldn't have spent all of Tony's childhood looking for him. Maybe then he would have been around more as a parent, maybe then Tony wouldn't have grown up thinking he was always second best, thinking he had to work to overcome the shadow of a fallen super soldier before he could get any respect.
Deep down Steve knew that wasn't true. Howard Stark was a lot of things, but Steve had never seen him as being a capable parent. He was too off-balance, too much like how Tony had ended up being. He spent months in the lab working without reason, building the next best thing had always seemed to be enough for him. Whereas Tony got his motivation from his faulty need for redemption or his desire to make the people around him happy, Howard got his motivation from himself. He just wanted to see what could be done, and he'd lose interest as soon as the job was accomplished. Steve had the terrible feeling that, regardless of his being there, Tony would have ended up with a bad childhood one way or another.
Still, it hurt Steve to think about the man before he had developed his aloof façade. Before he had discovered that it wound up hurting way less if you just didn't let people in, if you shoved everyone off before they had a chance to get anyone near you and filled your need for social interaction with people you could pay to do as you say. Sometimes he still wondered why Tony had bothered to change enough to let him in, he was nothing special. If anything, Steve was the one guy he'd never expect Tony to let in. After all, even if he believed Howard would have been a poor parent regardless, Tony certainly had to believe that Steve was the center of his dad's attention, and thus the reason he had grown up alone.
Steve didn't question Tony's decision though. He was afraid to. What if Tony realized that Steve was right and stopped talking to him? What if he left the Avengers altogether and the team began to split up? More importantly, how could he ever expect their relationship to continue if he brought up this point?
Because as much as it frightened him, Steve did want this relationship to continue. True, he had been raised in the 40's and still got a little nervous showing anything other than platonic feelings towards Tony when others were around (luckily Tony understood and they hadn't really told anyone they were together yet; it wasn't so much that they were keeping a secret as they were not bringing up what no one asked about), something about being with the genius felt right. He liked it when Tony smiled, because when they first met those honest and genuine smiles were so rare, and he liked knowing that he was somewhat responsible for putting that smile on the man's face. He liked having someone to take care of, even though it bothered him that Tony was so foolishly reckless with his life.
That was Tony's one absolute fault, Steve thought. He didn't care about himself at all. He didn't even think to look after he jumped, he just did things. There was no hope that they would work out, just him taking risk after risk after risk. He pushed his suit to the brink, even after Jarvis warned him that he was low on power. If he was told he couldn't do something he'd throw everything away to prove it wrong. He was absolutely careless when it came to his own life, and Steve just didn't understand it. It wasn't so much that Steve hated how Tony wouldn't eat or sleep on his own (although that bothered the super solider a lot, too), it was that he didn't think to keep himself safe when fighting. He didn't think he was worth keeping safe. If there was a grenade thrown at the Avengers, Steve was certain Tony would leap at it. The problem was, he wasn't sure if it'd be because Tony wanted them to live, or because he wanted himself to die.
Steve loved Tony. He wasn't ashamed to admit it, although he hadn't done so vocally yet. He loved just about everything the man was, he had the confidence Steve had dreamed about both before and after the serum (and he didn't need any risky drugs to make him that way, Tony's confidence was all on his own) and he was remarkably intelligent both naturally and due to his own hard work. The only reason Steve hadn't told Tony yet was fear. Fear that he'd be rejected, partially (although he was fairly certain Tony wouldn't look at him with disgust, he was fairly sure this wasn't just some quick fling to the man), but mostly fear that the words would frighten the man. Tony's fear of commitment wasn't a secret, and Steve wasn't sure how many times the man had actually been told those three words – and how many times they had been said to him honestly, without an end goal attached to them.
So Steve bit back his need to express himself. His need that Tony would wake up one morning and realize that people really did care about him, that Steve cared about him, that Steve would die for him. His need for Tony to understand just how much he cared about him. Instead, he worked to show his love through interaction; randomly popping down to the lab with a bouquet of roses, always making sure he made Tony' s favorite food when he was sure the man hadn't eaten in a while, holding him close at night and never denying him when he needed physical contact. He just hoped Tony could get the underlying message beneath all of his actions, and that one day he'd be ready to hear it put into words.