The habit begins in Cuba.
Or at least when she's in Cuba, sitting on a hotel bed while Strike Team Delta watches a terrible soap opera on the spotty television, and she's not sure what Natasha is laughing at but she thinks someone may have found out they're that tall bald guys son, or maybe she's just laughing at her partner. The three of them are waiting for a target that has a yet to arrive, a mission that won't begin for four hours, really, so she sits against the head board, Clint and Natasha on the other bed, and she talks to him on the phone.
She's in Cuba and he's in Iraq, in the last truck of a transport caravan, and he's ignoring his team because they're talking about who's mom makes the best home-style fried chicken, but he's not from the American South and his mum wasn't big on meat and he'd rather not join in.
"I'll make dinner for Monday," he tells his wife instead, because his mind is on food and it's bothering him a little that the men of his team talk about chicken like it was the only their mothers were good for. "You'll be home then, yeah?"
She nods on instinct and then says yes, of course, and that she's supposed to get in late Sunday night, probably after he's asleep.
"I'll wait up," he says and they both laugh because they know that him 'waiting up' really means he'll fall asleep on the couch with the TV on instead of in bed.
He has to go then, before the General finds him slacking off, and it's in the moment before he hangs up that his whole story of his morning hits her all at once and she feels a little afraid in her hotel room in Cuba, afraid of what might happen between now and when she wakes up next at home, on Monday afternoon.
And she wants to say she loves him but she's self-conscious about Strike Team Delta listening in, uncomfortable with her personal life becoming another piece of information to the team known for knowing everything, so instead she says, "Don't die out there, alright?"
"Back at you, sweetheart," he says with a small laugh. "I love you, Bobbi," he says next and she doesn't want him to go but he does, and she waits for the line to go well and truly dead before she hangs up.
She doesn't die in Cuba and he doesn't die in Iraq and on Monday he makes spaghetti for dinner because it's the only decent thing he can cook, not that she's any better in the kitchen, and they watch movies in bed for hours and get pasta sauce on the comforter and neither minds because they are alive and they are together and the night ends with a back rub and a goodnight kiss.
After a few missions and a few close calls and some drawn out stays in sterile white rooms, 'don't die out there' sticks: it's a reminder to stay alive, it's a promise to come home, and it's an 'I love you' with a personal touch.
When she's on the phone with her "friend" and he's yelling in the background, angry and pained and betrayed, and she's hiding around a corner and trying to stay calm and composed, and he says he takes them all back, takes back all those years of trust and faith, she's knows she's lost him, and it hurts in a way she wasn't expecting because maybe she was hoping the promises could stay. Maybe they could've kept not dying out there for one another forever; maybe they could've been fixed by words.
Maybe not.