Children are taught to ask for things politely and to say thank you when they get them. This is true whether they are asking for something from a parent, another child, or even God.
A lot of people never really grow out of a childish relationship with God, even if they do have good manners:
It usually doesn't take many tries to learn that you don't always get what you ask for from God. Our Faith teaches us that all things work for good for those who love God, so we may perhaps advance to the point of accepting God's will whether we like it or not:
Perhaps, though, we ought to separate asking God for things we want and thanking Him for things we get:
Here the "Prayer of Thanksgiving" that follows the "Prayer of Supplication" is a prayer of thanksgiving for the prayer of supplication; we thank God for allowing us to ask Him for things, for the grace by which we've just asked for one particular thing, and for whatever good He does in our lives in answer to our prayer.
But that's just a special case of the more general "Prayer of Thanksgiving" process, in which we thank God for everything He gives us. Breaking the supplication-obtaining-thanksgiving chain teaches us to be thankful, not just when we get what we ask for (like children), and not even when we don't get what we ask for (like philosophers), but when we get anything at all.
And this thankfulness is not the pro forma kind we offer when, say, a sales clerk hands us an item we have just purchased. God giving us something we've prayed for is just as much a gift as Him giving us something out of the blue. That it's something we've asked for may add to our gratitude, but as a circumstance of our gratitude, not it's essence.