While he's still nervous, still half-sure she'll regret it, he also knows if she tethered with anyone else he'd hate himself, and probably whoever else she chose. And she and Steven are suffering for the wait. The experiment's done, and this tethering thing is clearly affecting off-worlders, too. It's time.

So he doesn't even sound reluctant when he finally caves: "Yeah, I'm ready to call it. Steven, you're right. As usual." (Thank you!)