OK, I'm going to start off by saying that I HATE my headcanon for this. But my brain has decided that
- After they meet in the rain, they quickly get hot 'n' heavy with some apocalyptic sex. Maybe one or both have been pining, maybe not, but here they are so yeah, they're totally doin' it.
- They converge at Avengers HQ and hatch the plan to go have Carol punch Thanos and Get Everyone Back.
- This scene is right before they go. They realize that either they're going to be unsuccessful and probably die, or they're going to be successful and Clint's InstaFamily will return, so no more Clint & Nat sexytimes. Either way, it's goodbye.
In this picture they're both mad AF because Cap has hammered on their door and told them put on some goddamn clothing (Cap swears now, it's a thing) because they have to go save the world and also they've been in there for 17 hours with nothing but a box of protein bars and a case of water.
And Clint is telling her to hang in there a few hours, that as soon as they actually get on the spaceship they'll find a corner somewhere and get right back to it, and also he's going to sew all of Cap's boxer briefs shut.
Cap TOTALLY swears now! That clenched jaw in the first trailer says it all. He's seen it all. He's dealt with what he thought was it all. It was supposed to be enough. WHY IS IT NOT ENOUGH?
And as much as he's happy that Natasha is finding some happiness, there is a part of him that is irritated by any happiness. Any good thing that still happens from time to time because that's the human condition to be looking for, and celebrating, tiny good things. He had his tiny good thing. He lost his tiny good thing. He's pissed and he swears now.