I believe one of the big obstacles to having a thankful heart is an inability to embrace God's mercy and grace.
What do I mean by that?
Let me illustrate with the familiar story of the Prodigal Son. The Prodigal Son is not grateful at the beginning of the story but becomes thankful and is rejoicing by the end of the story. How does that happen?
Well, to my point, when we first see this young man we see someone who doesn't know how to handle grace or mercy.
First, he abused the graciousness of his father by demanding his inheritance. His father was not obligated to grant this unusual request but he chose to do it because he loved his son. By doing this he broke tradition and social custom, bringing disgrace to himself and dishonor to his father.
Also, he abused mercy.
How did he abuse mercy?
The inheritance system was set up to protect young people from getting too much wealth and responsibility too soon in their lives. It was a form of mercy.
When you consider how he abused grace and refused mercy to get his own way, you know it is not likely to end well. He was ungrateful in his demand, and he was ungrateful in what he received so how do think he is going to turn out?
Once his demand was met and he got what he wanted from his dad, he took the money and ran! He ran to spend it on wild parties and wild women. unfortunately, when his money ran out so did his friends. Soon he found himself broke and lonely and hungry. He was in a world of hurt scuffling with swine for their scraps.
What if this young man had been grateful in the beginning?
What if he had honored the grace and mercy of his father from the start?
Imagine how much different the story would have been!
A truly thankful heart has learned to receive the mercy and grace of his Father.