The Probability Argument for Desires

Lewis: If I find myself a longing which this world cannot meet, then it probably means that I was made for another world. Lewis wasn't dumb there. He knew that it's not a demonstrable proof. It's a probability argument. This question's a little long, so I'm just going to pare it down, so I hope you don't mind. Do you think every human being is capable of loving? What about people who are born with medical deficiencies? Would the Christian gospel imply that that person is not actually human because they cannot love God in order to find satisfaction?