He could've slept in the cold. He can sleep in anything, a nice side effect of being in special forces. Unfortunately, he knows all too well the possibility of freezing to death in a temperature like this, and he doesn't know how well the cabin will stand up to unexpected temperature drops.

So here he is coming downstairs, seeing Loki huddled by the fire.

"It's cliche," he says, "but we gotta share body heat. Otherwise, there is a very real possibility that one or both of us will die." Maybe not a really big possibility, but it's there, and that's enough for Clint.