It is not the tenth time that one of them almost gets killed that finally does it. Or the way his eyes reveal everything he wants in another one of those discussions about others that are really about them. It isn't even the latest undercover mission where they once again sleep side by side for many nights.
It has been so long coming that the scales didn't need to be tipped by the heavy weight of a grand event or a near death experience. The decision that brought her here on his doorstep on this perfectly ordinary day has evolved so slowly and naturally that it had taken her a while to realize it had been made long ago. The decision that it is time to reach for what she really needs. The knowledge that she deserves it. The acceptance that even though nothing is inevitable, this might be as close to it as it gets. And that this is worth risking everything they have for, although it is clear that there is really nothing at risk.
Because even though she is afraid to fall, she knows she can trust him to catch her. And so it is that every trace of hesitation is gone as she stands in front of his door, raises her hand and knocks.