He can feel her fading.
It's been a slow death. It started in Budapest, though neither of them realized it at the time. Looking back, it holds a cruel irony – for him, it was the climax, the start, of a beautiful love. He thought she had finally put away her adamant, "Love is for children."
But it became clear that what he considered a starting place had in reality been the end.
He had always known they would end in tragedy. He had known it from the moment he stared down the length of his arrow and knew he couldn't kill her. He fell for her then, and he's never come back up.
New York. They fight together, backs against each other's, as one. Together, unstoppable, perfectly connected in every way. But something has shifted, and it started a long time before Loki took control of his mind. He's only now realizing that it runs much deeper than that.
He no longer always knows what she's thinking. She doesn't come to him when she can't sleep. In fact, she doesn't come to him at all.
She doesn't say I love you anymore. Not even when he says it against her lips, desperate to hear her say them back. She's never lied to him. But just this once he wishes she would.
He wakes up one morning and goes to the roof like he – like they – always do, staring out at the skyline of the city to watch the sunrise. For the first time in a lifetime, she doesn't join him. He knows she never will.
It's been a slow fade. Maybe if he'd noticed it sooner, he could have stopped it. Maybe if he'd just stopped, that day in Budapest, and thought about it…
Maybe he got too close, and she got scared. Maybe that's why he woke up this morning and she was gone.
In the end, they will always remember Budapest differently.