Jesus taught his disciples gratitude though his teachings, and more importantly, by his actions. It is in thanksgiving and gratitude that he helped the disciples remember that God was the source of all their blessings.
Giving thanks to God kept the disciples, and keeps us focused on God and away from all of our own accomplishments. Some people even can arrive at a point where all they can do is appreciate God for the gift of their lives.
I'm reminded of the story of a man who traveled a great deal. He missed his children a lot when he was away, so it was his habit to bring his little ones a gift when he came home. So he would be gone, and when he came back he would have a nice gift for his kids. After a while it seemed to him that when he came home, the kids looked forward more to the gift than to seeing their dad. Now isn't that funny? Is that us? Are we looking forward to what God does for us, all the gifts He gives to us? Do we think so much of the gifts that we forget the giver?[i]
Thankfulness is a choice. We make up our mind, and we determine that we're going to have a thankful attitude even in times of struggle, trouble, sadness, sorrow, and in view of church history, sometimes even persecution.