The parashah begins with a general commandment; to be holy. According to the Ramban, being holy means to not give into our animalistic impulses, to rise above them, even when something is technically permissible according to the Torah. In other words, we are being called upon to restrain ourselves from indulging in our base desires and to strive for a higher moral standard.
(רמב"ן ד"ה קדושים)
Breaking Nature
The Rebbe Reb Elimelech of Lizhensk writes: הָאָדָם לֹא נִבְרָא בָּעוֹלָם - רַק לְשַׁבֵּר אֶת הַטֶּבַע - the only reason why man was created in the world is to overcome his inborn nature, to challenge and surmount his natural tendencies. For this reason, he says, one must work intensely on rectifying his middos, for this is the sole purpose of his creation.
The Rebbe Reb Elimelech advises how this is done, and sets a few examples how to apply this method to one's bad natures. For instance, he says, if someone is born with a stubborn nature, they should take up the challenge of doing the exact opposite of what they wish to do for forty consecutive days. If someone is naturally lazy, they should make a conscious effort, for forty days, to do everything enthusiastically, including mundane tasks like getting dressed, washing their hands, and cleaning their body. In the same way, if someone is shy, they must try to overcome their shyness by davening with a loud voice for forty days, with forceful movements etc., until with help from Above, his natural reticence which sometimes keeps him back from doing what is necessary, is rectified.
(צעטיל קָטָן אות טז)