Gratitude is the appreciation we feel when someone does something kind to us or for us. There are two factors that effect the intensity of gratitude. 

First, there is the deservedness factor. Say you do a favor for your neighbor. A month later, your neighbor does a favor for you. You are grateful, but not overwhelmingly so, because you deserved his help to some degree. However, if you have been an unkind or selfish neighbor, and yet your neighbor provides help when you are in need, you are extremely grateful precisely because you know you are undeserving. 

Second, there is the generosity factor. Imagine you pick up the mail for your neighbor for the three days he is out of town. When he gets back, he gives you a hundred-dollar bill. You might protest, “This is too generous!” He insists, and so you are very grateful.

Apply this to God. Deservedness. How deserving are we of his blessing? What does God owe us? Generosity. What has God done for us? What blessings has he given us now? What blessings has he promised us in eternity? As we answer those questions, we are stunned by the undeserved generosity our God has shown us. The effect? We overflow with gratitude.