The Israelites had just been delivered from slavery when they began to grumble against Moses and Aaron: If only we had died by the Lord's hand in Egypt! There we sat around pots of meat and ate all the food we wanted, but you have brought us out into this desert to starve this assembly to death.
There are two lies that are believed when we begin to grumble.
The first lie is God is not wise. The Israelites really believed they knew better than God as He had made a mistake bringing them into the desert. If God really knew what He was doing, He would have left them in Egypt where slavery was so much better! Really? Remember the bricks? Here is the truth about the person of God - He is all wise, even when we do not understand His ways. We become convinced that God does not need to work on us any more and really, if we are happy, he should leave us alone. But God is interested in change and grumbling resists change. A thankful heart knows that in God's great wisdom, trusting that He is bringing us to a better place.
The second lie is God has abandoned His people. Would God really leave His people to die in the desert? Did He not make a covenant with them? Did He not just deliver them from a powerful army? Did He not declare "I am the Lord"? We have so many assurances God is leading us and will never abandon His people. Grumbling forgets the promises and person of God.
Grumbling is a matter of the heart, resulting in a sinful view of God. Knowing God and holding onto His promises will produce a thankful heart as we know all that is given is far more than we have ever deserved.