From The Quiet Place: Daily Devotional Readings by Nancy Leigh Demoss:

Every hint of ingratitude is an indicator of pride in our hearts, the idea that we deserve more than we’re getting, that we’re inherently worthy of being treated more generously by God or others. Oh, we think we deserve so much. But how thankful we should be that God has not given us what we deserve, choosing instead to extend mercy and grace. Pride is the father of ingratitude and the silent killer of a grateful heart.