Psalm 107:8, 15, 21, 31 (NKJV)
If we hear something once it might or might not be important. If we hear something repeated two or three times we usually take notice. But, when we see something repeated four times it is obviously something we need to pay attention to. In Psalm 107 this one statement about being thankful is repeated word for word four times, that is significant! This Psalm is filled with reminders of good things that God has done and it repeats the idea that it would be right for people who have experienced God's goodness to give thanks to God for all of the wonderful things He has done.
A good example of this is seen in the story of Israel coming out of Egypt and traveling through the wilderness to the Promised Land. When you read the story of this journey, instead of seeing Israel giving thanks to God as they should, you see them complaining about how bad the wilderness is and how wonderful it was back in Egypt (where they were slaves), how they wished they had better food to eat (even though God's provision literally came out of nowhere to feed them), and how every difficulty brought crying and complaining (even though time after time God delivered them).
It is easy for us to say, "Well, if I was in that situation I would be thankful". But in reality we are often just like the people of Israel, instead of being thankful for what we do have we wish we had something even better. Certainly there are times when things could be better than they are but we have the choice about what we will focus on. We can be thankful for what we do have or we can complain about what we don't have. If we take some time to really consider the good things that God has done in our life it will help us to be thankful. Make the choice today to be thankful and focused on the good things you have received and not be a complainer about what you don't have. It is good to give thanks to God!