“Is River Song your wife? ‘Cause she’s someone from your future. And the way she talks to you, I’ve never seen anyone do that. She’s kind of like, you know, ‘Heel, boy.’ She’s Mrs. Doctor from the future, isn’t she? Is she going to be your wife one day?”
The Doctor’s little smile in this gif is for one reason: he’s learned something new about his relationship with River Song. See, River carved the Doctor’s school name on the cliff: Theta Sigma. Not many people know the Doctor’s name from when he was in the Gallifreyan Academy, so it speaks to the intimacy of their friendship/relationship. He now knows that he’s confided in her at least to that extent, which is why he doesn’t hesitate to follow her instructions.