Fulton J. Sheen wrote: "An interesting phenomenon in children is that gratitude or thankfulness comes relatively late in their young lives. They almost have to be taught it; if not, they grow up thinking that the world owes them a living."
A friend once told me that she didn't want to force her son to say "Thank you" unless he really felt like it saying it. "If I teach him to say 'thank you' when he doesn't feel thankful, I'm teaching him that it's OK to be a hypocrite."
That's not even close to what gratitude is. Our feelings have nothing to do with why we express it. Gratitude is not an emotion, it's an action. The act of saying "thank you" is for the benefit of the other person. It's about their feelings, not yours.
The same is true when it comes to say "Thank you" to God. Thankfulness is the proper response to the goodness of God. We say "thank you" because he is good, not just because we happen to feel good.
That's why the Psalms so often refer to the "sacrifice of thanksgiving" -- it's an act of obedience, not just an emotional outburst.
David said, "I will offer to thee the sacrifice of thanksgiving, and will call upon the name of the Lord." (Psalm 116:17)
Like children, believers need to learn how to be thankful. Most of the time, when we consider all the good things God has done for us, we'll feel thankful. Even when our feelings don't cooperate, we need to properly express gratitude, offering God a sacrifice of thanksgiving for the kindness and mercy he has shown us.