There were a ton of things Ben would have told her about where he was from, but that one... if felt wrong to tell her. Not because it wouldn't be what she wanted to hear, but because it was. What was it Cas had always said when he was a kid? Something about it being important for Ruby to have things for herself. Ben, at that age, had always guessed that it meant Ruby needed to 'get a life', but now, at this age and looking back? He could see what Cas had meant. If he told Ruby, and she latched on to a future with Cas to keep her going, maybe she'd never get better for herself.
"I'm gonna break the future if I tell you, you gotta know that," he said, still quiet with an edge of desperate. If he fucked this up, it would ruin the future. Hers, his, everyone's. "The future's good, Ruby, good for you. That's enough to know, isn't it? Can't the rest be a really kick ass surprise?"