The one thankful leper didn’t have his leprosy cured because he believed hard enough. Jesus commends him because he saw in Jesus the merciful God. God had shown pity on him, and therefore the leper was glad. I imagine they all were glad, because they were healed. But the thankful leper was glad not just for his healing, but because he had found God. He wanted to be with him. This kind of faith ceases to be about the believer and becomes all about the object of belief. The Christian, then, never talks about his own faith, but the Christ in whom his faith is.