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''You shall be holy'' [Kedoshim 19:2]
There is a principle in Chassidic thought that ''the higher something is, the lower it falls''.
For this reason, a person reaches the highest levels of spiritual greatness, not through intellectual endeavours alone, but by involving himself in the physical world, observing the mitzvot and helping others to do likewise.
Thus, at the literal level, one actually fulfills the command ''You shall be holy'' by refraining from the lowest and most debased acts [i.e. forbidden relations - see Rashi v.2]. For the route towards the highest degree of holiness, becoming holy like G-d [''You shall be holy, because I, your G-d, am holy''] is through refraining from the lowest of acts, because ''the higher something is, the lower it falls''.
This also explains why, at the minchah prayer on Yom Kippur, the holiest day of the year - before beginning the Ne'ilah prayer, the climax of the day - the Torah portion that is read discusses forbidden relations. For it is through restraint from the very lowest of acts that one reaches the very highest degrees of holiness.
Source: Lubavitcher Rebbe: based on Sichas Shabbos Parshas Kedoshim 5725