[Bucky doesn't need to be a telepath or a freakishly observant detective to know Ben's probably not taking this information well, despite outwardly taking it better than he would have expected him to, so his immediate priority was making sure he wasn't going to completely flip out over this.]

Hey, no, this isn't your fault, alright? I don't know why it happened, but it's not because of you. [He snorts quietly, making a little bit of a face.] Hell, maybe I'm the one who screwed up. This just happens, sometimes. I don't think anyone knows why.