“Nachmanides explains that the mitzvah of ‘Ye shall be holy’ means that a person must sanctify himself with those things permitted to him so that he will not be ‘a degenerate with the Torah’s sanction.’
Each of us must make moral choices as to how we will conduct ourselves.
The Torah permits marital relations, yet a person can, while acting wholly within the framework of the law, indulge a greedy lust born of unrestrained desire.
The Torah allows the consumption of meat and wine, yet a person may, while eating only kosher foods (even simple food, as he exercises proper table manners), become obsessed with gratifying his palate.
Or someone may be honest, yet immersed in the pursuit of money with such utter self-abandon that it becomes his personal idol.
In this sense, the Kotzker Rebbe states that a person can commit adultery with his own wife.
Our greatest test, therefore, is making the permissible holy.”
From Understanding the Tanya, p.87, by Rabbi Adin Steinsaltz