“If you are ungrateful Allah is rich beyond need of any of you and He is not pleased with ingratitude in His slaves. But if you are grateful He is pleased with you for that.” (39:8)
Ingratitude is another of the bad qualities whose presence prevent people’s hearts from awareness of their Lord. It manifests as discontentment with one’s lot in life and complaint about one’s circumstances. This demonstrates deep ignorance of the nature of existence because it means you think things should be other than the way that Allah has decreed they will be and also implies that you think that you could do a better job than the Creator of the universe.
The truth is that everything we have is a gift from Allah, our existence itself and all our sustenance, but we tend to attribute the things we like to ourselves and anything we dislike becomes a source of complaint. “If We let man taste mercy from Us, and then take it away from him, he is despairing, ungrateful; but if We let him taste blessings after hardship has afflicted him, he says, ‘My troubles have gone away,’ and he is overjoyed, boastful.” (11:9-10)
It is worthy of note that the Arabic word for ingratitude is kufr which is the same word often translated as disbelief. This key Arabic term means in its root to “cover over”, and in the context of disbelief “to cover over the blessings of Allah” i.e. to be ungrateful. In extremes it means also to reject. This rejection of the Divine has as its beginning point simple ingratitude.
(Islam – Its Basic Practices and Beliefs, Abdalhaqq Bewley)