The Doctor's thinking along similar lines as well. Before Toothless came along, he was on his own, except for the instances when he passed another wagon. But passing someone by doesn't really count as companionship, so he's glad Toothless is here. When the dragon sits beside him, he reaches out to give him a scratch behind his ears.

"You know, whether you're traveling along the Oregon Trail or you're just going for a walk somewhere, having someone alongside really makes the whole thing better. I'm glad you turned up when you did."