He can feel the tension, pulling Daenerys taut like a bowstring. In his mind, Jon can almost picture her standing on the edge of a great height, but instead of wanting to catch her or pull her back, everything inside of him is demanding he push her right over. And so he does everything he can to do just that, reaching up as he does to find Daenerys' hand, instinctively lacing his fingers with hers so she can squeeze as hard as she likes.