Numbers 11:1 "Now when the people complained, it displeased the LORD; for the LORD heard it, and His anger was aroused."
Pretty self-explanatory. Why should we complain when we have so much to be thankful for. When we complain, we are saying that we are not satisfied with the way the Lord is providing for us and guiding us, thinking we could do better or that we deserve better. Looking back over my life, I know that God's ways are perfect. He has blessed me abundantly and I have NOTHING to complain about. Blessed be His name.