Doctor, you and all of your different versions do keep him thinking... and he does love that. He never quiet knows what kind of surprise will be thrown at him next! He sees the Doctor as a kind-hearted person and a good friend who helped him out when he was having a moment of feeling too far behind the times to be of any real use. That was a gift he is still grateful for. He would love to just sit down with the Doctor sometime and pick his brain, all of this talk of time travel is absolutely fascinating!