They forgot. In the midst of the hot, dry, desert wilderness, they forgot God's goodness. They wished He had not done what He had done in bringing them out of Egypt. Even though they were in slavery, they lost sight of the Promise.
Forgetfulness breeds hopelessness. They forgot God took them out of slavery to give them a good land. A profitable land. One with wheat and milk and honey.
We must remember the whole way the Lord has led us. Out of the bondage and despair, from darkness to marvelous light. Maybe you feel like you are in Egypt. Or in a dessert. You know what He has promised but you've forgotten. And so, you're wandering, chewing on manna, questioning His goodness.
Remembering and recounting God's faithfulness lays the grounds for thanksgiving and a life of joy in Him. In His goodness. Rest in that. There's hope there. The desert is hot and dry and hard. Come to the promise - Jesus, the promised one, and be refreshed.