Then one of them, when he saw that he was healed, turned back, praising God with a loud voice; and he fell on his face at Jesus’ feet, giving him thanks. Now he was a Samaritan. Then Jesus answered, “Were not ten cleansed? Where are the nine? Was no one found to return and give praise to God except this foreigner?” Luke 17:11-18
Relatively few people who receive the goodness of the Lord return to give Him thanks for what He has done. That does not keep the Lord from doing what is right for us. He healed all ten of these lepers according to their request – not just the one who was thankful. However, there was only one out of the ten that was made "whole."
The Lord desires that we prosper in spirit, soul, and body. He wants us to be whole – not just healed. Part of the reason God meets our physical needs is to prove to us His willingness and ability to meet our emotional and spiritual needs. The Lord is concerned about our temporal needs (Mt. 6:30), but He is even more concerned about our eternal needs. All of these lepers needed physical healing and the Lord was moved with compassion and met their need.
He was also desiring to meet their spiritual needs, but only one out of the ten came back for that.
Being unthankful is always a sign that self is exalting itself above God. A selfless person can be content with very little. A self-centered person cannot be satisfied. Thankfulness is a sign of humility and cultivating a life of thankfulness will help keep "self" in its proper place.
Thankfulness to the Lord for what He is and what He has done is a very important part of the Christian life. One of the many benefits of thanksgiving and praise is that they keep us from being "self" oriented.
Giving thanks is a totally unselfish action and is a key to relationship with the Father that makes us "whole" and not just "healed."