Harm No. 9: Ungratefulness
Casting evil glances brings about ingratitude within the heart. When a person is looking at so many faces and features then his own wife does not remain attractive to him. Hence, he becomes ungrateful thinking to himself that he did not get a good-looking wife.
And if she is good-looking, then he thinks to himself that she could have been more beautiful. That woman which appears more attractive to him than his own wife, then even if his own wife is beautiful, she is still not as attractive as the other woman. In this way, he is becoming ungrateful of Allah's favors.
As for the one who guards his eyes out of Allah's fear, and does not look at other women, than even if his wife is not so beautiful, she is still attractive to him and in this manner he remains grateful to Allah for this favors.