The spirit of complaint is born out of an unwillingness to trust God with today. Like the Israelites, it means you are spending your time looking back toward Egypt or wishing for the future all the while missing what God is doing right now.
Discontent
Arthur W. Pink
Satan is ever seeking to inject that poison into our hearts to distrust God’s goodness – especially in connection with his commandments. That is what really lies behind all evil, lusting and disobedience. A discontent with our position and portion, a craving from something which God has wisely held from us. Reject any suggestion that God is unduly severe with you. Resist with the utmost abhorrence anything that causes you to doubt God’s love and his lovingkindness toward you. Allow nothing to make you question the Father’s love for his child.