One thing everyone knew about Natasha Romanoff is that she thought that love was childish. But what they often missed when she said that was the envious way her lips would set, how her face would, for just a fraction of a second show a deep longing, before clearing completely of any emotion and her mask settled back in. Love might be for children, but most people remained, at least a little bit childish at heart, something she felt she had lost long ago.
Natasha Romanoff, on top of thinking that love was for children, did not think that love was about finding "your better half." She thought that if everyone would focus more on bettering themselves, than on finding their "better half", the world would be a much better place. She was a firm believer that better halves didn't exist for people, that no one would magically make someone a better person.
These two things, guidelines, rules, she would even call them, were the way she often saw through relationships and love. With these two rules, and watching the different ways in which people reacted to them, made it easy to see through a master spy's advances for courtship, and the most concealed hatred for an unfaithful spouse. Natasha Romanoff had rules for reading people, and they were never to be broken.
Except when it came to her two rules on love.
She herself broke the first rule. She didn't think she had a child in herself before joining SHIELD; there was never any happiness, any enjoyment, any love. There was nothing after she left the Red Room but repaying debts, cleaning the red from her ledger, nothing but redemption.
So when Natasha Romanoff found herself running into a burning building, to save her SHIELD handler, Phil Coulson, she was surprised to find that she was doing it to protect him, to protect someone she loves. Coulson was the first thing she could even come close to calling a parent, and she felt a strong need to protect her pseudo-father. After the first epiphany of love, and she recognized the feeling, Natasha Romanoff realized that she loved a lot of people. Clint Barton, the man who enlisted her into SHIELD and her future partner. Nick Fury, with their mutual trust and dependence on each other, as well as his protective actions toward her. And once the Avengers joined the list with the others, Natasha Romanoff loved to admit it. Her rule that love was for only those with childish hearts was broken.
The person to break the second rule really pissed her off. When she realized that her first rule was broken, in her first year of working with SHIELD, she did not expect her second one to last, but it did, through her remaining years as a SHIELD agent. Actually, the best SHIELD agent, if anyone was keeping score (and she was). But a certain mission came up, and flushed her entire career of flawlessly reading people down the drain.
The "prone to self-destructive tendencies" part of Tony Stark's evaluation was the only thing she had gotten right about the man. If he was compulsive, it was a calculated coverup and retaliation against an attack, or it was to protect someone he cared about. He was only prone to self-destructive tendencies because he tried to save literally everyone. Natasha Romanoff is now relatively sure that Tony Stark would jump in front of a speeding truck if he could save a puppy in the progress. And now that he was a person that she cared about, she saw that the narcissism was a mask for the fear of everything he had worked for being taken from him, and used to harm others. The narcissism and selfishness was mostly to protect others, to keep himself and the things he made out of the wrong hands.
And even after this jerk had proven Natasha Romanoff wrong, and shown she was not perfect in her ability to read people, he has to go and break her second rule.
There were no doubts in anyone's mind that Tony Stark was annoying. It was just how and to what degree you found him annoying. This far in the game, to Natasha Romanoff and the other Avengers, Tony Stark was a lovable little brother kind of annoying. The rude quips and jabs no longer bothered Steve Rogers, the lack of sensitivity in regarding Loki no longer enraged Thor Odinson, and the brutal understanding of Clint Barton's guilty consciousness no longer drove the archer away.
Everything that made Tony Stark the annoying piece of shit he was no longer got under any of the Avenger's skin, and he became more endearing to hear his snide remarks. But, man, was it still annoying.
What Natasha Romanoff really didn't understand was how Tony Stark truly did find his better half. He was still himself around the fabulous Pepper Potts, but it was toned down, softer, sweeter. Everything he did around Pepper Potts was to protect her and keep her safe. Natasha Romanoff knew that if Pepper Potts were to no longer be in his life, Tony Stark would not last a day. Tony Stark depended on her, and without his other half, he would cease to function. There was not a thing he could do without his other half; it was like trying to start an engine without any pistons, it just wouldn't work.
And it was after all of this contemplation, Natasha Romanoff made two new rules for love. To feel love, you just had to have a heart, not necessarily a childish one. This was true, she determined, because she could love without a childish heart, and Tony Stark could love with his heart, albeit damaged heart. His damaged heart needed another half to be complete though. Her new rules for love became that to love you needed a heart, one that was not surrounded by dangerous shrapnel. Her second rule became that even with his shrapnel surrounded heart, Tony Stark could love, because the proof that he had a heart was named Pepper Potts.