Natasha Romanov loses people. This is a fact, one of the earliest she's ever learned in the Red Room. You lose people; you do not gain them. Anyone who is in your life for long enough turns into either a threat or a liability is the second fact the Red Room teaches her. Therefore, you do not let people get close to you is what Natasha learns.
Clint Barton is the first person to prove both of those facts wrong.
There are always exceptions to rules, of course, but not this one. Never this one. In the Red Room, you do not get to keep people. So, Natasha spends the first three years of their friendship trying to push Clint away and Clint spends those years pushing back.
He breaks down her walls eventually and they become the most important people in the world to each other. No one else can share their bond and no one else attempts to.
Phil Coulson is the second person to prove the rules wrong.
He's there for both of them since Clint's recruitment into SHIELD, is the person that fought for them when Fury wanted them out of his organization and is the voice in her ear through countless missions and debriefs.
She almost doesn't realize how comforting his presence is until the day Clint is shot outside of Moscow in what used to be her home territory. Natasha almost lets her walls down, almost breaks in front of Phil, but she keeps her walls up and he talks her through removing the bullet and Clint lives.
After they come back from the mission, while Clint is hovering between life and death in SHIELD medical, she crumbles. Phil is there, like always, a steady rock that Natasha clings to during her storm of emotions. He watches her cry and then reassures her that the rules are wrong.
They form a perfect triangle, and Natasha allows herself to believe again. They become SHIELD's most valued team, and those years are Natasha's happiest.
Then Loki comes to Earth, the Tesseract is stolen, and in less than a week the rules are again proven right, and she loses Clint and Phil both. She figures it's payment for her stolen happiness, and settles her long-practiced mask into place.
She holds it together through the battle of New York, puts her walls up and stifles her emotions. Even though her heart feels like breaking and she wants to scream until her throat is raw, she fights, taking out her anger and frustration on the Chitauri. She tears them apart mercilessly as revenge for Phil and Clint, until she no longer feels broken glass every time she swallows, and her eyes are completely dry.
That night, she cries in Clint's arms. He's back, against everything she's been taught, against every code and rule and law, he's back. Some of her heart comes back together again, but she still guards herself, because even if Clint came back, Phil is dead and the rules are right about that.
The Avengers fall apart after the Chitauri attack. Tony goes back to the Tower with Bruce and doesn't contact SHIELD. Clint and Natasha go back to SHIELD with Steve, although he leaves the next day with his suit, a duffle bag, and a too-thin folder, and Thor leaves Earth with Loki.
None of them contact each other. Despite Fury's hopes and misplaced belief, the Avengers Initiative failed.
Natasha and Clint are given a mission two and a half weeks after the Chitauri invasion is over. It's simple – almost insultingly so – but Clint's eyes are frightened behind his cheap plastic sunglasses and Natasha keeps checking her weapons she knows are there, so perhaps it's justified.
They find the target, a low-level Hydra informant, and take them out without any trouble. Clint makes a joke about it being a vacation for them, and Natasha smiles, the expression feeling foreign. Then, they get a message. It's from an untraceable number and it reads: this is Stark. Come to the tower asap. Don't tell Fury.
Natasha loses people and so does Clint, but there's a measure of trust between the Avengers-that-no-longer-exist, so they go to New York instead of SHIELD headquarters and are standing at the Tower's doorstep within fourteen hours.
They're surprised by what greets them – Steve, looking like death warmed over, Bruce, his skin slightly green, and Tony, looking more exhausted then all of them put together. They gather at a table in one of the meeting rooms, and Tony produces a stack of files. "Take a look," he orders.
"Where'd you get these?" Steve asks, looking at them like he did the weapons from Project Pegasus.
"I downloaded them from SHIELD and then offloaded them to several secure servers and printed them out for an added layer of security," Tony says, and Steve nods like he understands every word. Maybe he does. Natasha wouldn't know.
"What is it?" she asks, leaning almost imperceptibly towards Clint.
"Take a look," Tony invites again. There's anger under the friendliness, but Natasha knows it isn't directed towards them. It's the expression she's seen on Phil and Clint's face, and felt on her own, when people they care about are hurt. She wonders briefly if the Avengers could have worked but abandons the thought. They failed once and she isn't sure that she wants to give them another chance.
Bruce is the first one to take Tony's offer, and he drops the file almost as soon as he picks it up. He looks at Tony with desperation, and Tony nods.
"You know where the room is, Big Guy."
Before he's done speaking, Bruce is gone, and less than a minute later Natasha hears a roar that brings with it memories of being trapped and running through the guts of the Helicarrier with nowhere to go and danger behind her. She flinches, and Clint's hand finds hers under the table.
Steve is the next one to pick up the dropped file and he reads it quietly before passing it to Clint and Natasha. "Is this true?" he asks Tony in his stern, strict, 'no bullshit, I'm Captain America' voice.
Whatever the answer is to that question, Natasha doesn't hear it, because the rules have just been proven wrong twice. Coulson's picture stares up at her, the red Deceased stamp on his file crossed out in blue pen and written above it: Alive.
She squeezes Clint's hand under the table, trying desperately to blink back tears. She can hear Tony saying, "You don't have to say anything," to all of them, and it's quiet and honest, not the usual bombastic front she's used to from him.
"Thank you," she whispers, still staring at the folder. Alive.