“You shall be holy for I, Hashem your God am Holy.” (Lev. 19:2)
Why does God instruct us to “be holy for I am holy?” Why not just tell us to “Be holy”?
First let’s try to understand what “holiness” (kedoshim) means. Commenting on this verse, the Ramban explains that holiness in this context means separation, even from permitted pleasures. We must exercise self-control when partaking of allowable, physically satisfying activities such as eating, drinking, and marital intimacy. Otherwise we may be tempted into sordid overindulgence, which leads us far from holiness. We can eat without being a glutton and we can engage in sexual intercourse without becoming a slave to our lust.
The Torah provides a reason for us to be holy: because God is holy. Indeed, connecting with God is the reason for all the commandments that we follow. Jews refrain from eating pork not because pork is unhealthy or tastes bad, but because God commanded us to eat only kosher animals. Without a strong reason for avoiding temptation, we are likely to fail in our efforts. Instead, we should act for the sake of Heaven because that is how we become holy, as our God is holy.