We humans have a bit of a problem.
We don't exactly like staying on one path for too long. Survey human history and you'll see a pattern of disatisfaction develop, individuals and nations who are happy with their lot and then begin to become dissatisfied, only to start looking for something else, the next thing to take them to bigger and better places.
Dip anywhere into the Old Testament and you'll see a people completely dependent on God. And yet, time and time they turn away from Him. From the One who provided them with everything they needed; a land to call their own, an identity that set them apart, even food on their plates for the better part of 40 years (even if the menu plan may have gotten a little tedious after a while...)
So why did they turn away? Why turn away from the One who is very clearly providing at the time of your greatest need.
Because they forgot.
Time moves on, a pattern of life developed and they forgot the very simple fact that they couldn't do it on their own.
We are no different from these ancient people. We turn away from God and start doing our own thing when we forget that we need Him, when we forget that without Him our existence is finite. We think that we can go our own way, do our own thing, forgetting that nothing has ever existed that has not been wholy surrounded and upheld by the presence and grace of God.
When we forget to say Thank You, we forget what we have been given, our need of and dependence on the one who is the generous and merciful Giver.
Graditude to the Giver keeps us where we should be, in a place of dependence upon the only one that we can truly depend on.