He is laughing for the "great mentality" part i guess. And he is right, Santon has VERY weak mentality. He has the talent to be the best full back in the world, but his mental issues are not behind him, and they are the main reason he is so average.
If mentality is the only problem, then I expect him to become an absolutely top player at Milan or Juve. They know how to get the best mentality out of a player.
He wanted that goal since his first season with us, I remember how sad he was when he missed that shot after a great run and a nice one two with Ibra against Fiorentina (or was it Genoa?) in his first season. Good that he finally did it, congrats.
2010: "I will not go to Milan out of respect for Inter"
2013: "For other teams I don’t know, but for Milan I’d seriously think about packing my bags again. Inter? I owe them a lot. I was happy. But it’s not my fault if I was born a Milan fan.”