You shall be holy to me, for I the Lord am holy and have separated you from the peoples, that you should be mine.
God is holy. That fact is one of the most emphasized of His attributes in all of scripture. It is on display front and center in the book of Leviticus. God's holiness moved Him to set His people Israel apart to Him in holy living. And He expected His people to live differently than the pagan nations around them as a result. Personal holiness is all about that unique relationship with a holy God.
I want to live as God's set apart person in Christ. Jesus clothed me in His righteousness. I am set apart as His in the Church -- the people that are simultaneously the Body of Christ and the Bride of Christ that He loves and possesses. We are to be holy... I am to be holy... because Jesus is holy. This Leviticus principle of "be holy because I am holy" is repeated in the New Testament for good reason. I also live in a pagan culture that needs to see the beauty of God's holy love and life in the Church. We are the people that will show Jesus to them. And He will be known as holy.