I really hate that Luke 17:11-17 has become the great "thankfulness" text. You need to be thankful. See how few people are thankful - you need to be thankful.
All law - when this text is really about nothing but Gospel. Christ is worn and tired from travel - yet He still hears the cries of the Lepers and heals them. 9 of the lepers are short on thankfulness, yet they are still healed, because Christ is merciful. Their lack, their folly, even after receiving mercy is shown - yet Christ remains merciful.
He is merciful - go on with your life. He will heal you - go on with your life. What a comforting text!
And yet -- how many people will wag a finger and say, "Have you been thankful enough to God today?" No, I haven't - but He has been more than merciful to me, so stuff, you preacher of the law! Thank God for how holy you are, how you are so much more thankful than me - I will simply continue to call out, "God, be merciful to me, a sinner."
(Note: In our history study, we are starting to look at the rise of Pietism and Rationalism -- this means I may be in a very cranky mood against any and all forms of legalism, Christian self-help, works righteousness, or "christian" arrogance. You are warned)