Luke presents us with an account of people carrying out Biblical injunctions. That is, since they are lepers, they are keeping themselves separate from everyone else and also calling out a warning so no one will inadvertently come near them. Further, when they are cured, Jesus tells them to head for the temple so a priest can certify that they are no longer lepers. (That's also in the Bible. You can look it up in Leviticus 13:35-45; 14:2-32).
Nine of the ten who have been healed follow these instructions. However, one does something else. He returns to Jesus, thanks him, and gives praise to God for his healing.
Jesus asks why the nine others did not return to give thanks to God and points out that this one who did is a foreigner.
They all had been suffering. They all had turned to Jesus for help. They all had faith that Jesus could heal them--even the foreigner. And all were healed. Jesus then tells them all what to do next. Nine do it.
Yet, Jesus holds out for praise the one who returned to him for thanks. Sometimes, we have something to learn from outsiders.