He'd hardly had time to react when she did that. Her hand was in his and her lips pressed to his fast enough that Thor almost didn't process it. But there was no mistaking that feeling, that energy and the literal spark that went between them as he pulled back after just a moment. The electric charge faded and, on theme, the music changed to something a little slower and more fitting. Thanks, hotel ghosts!
That made Thor wonder. He questioned things to himself for a second. Then he nearly mumbled, "Diana, I..."