it is written: "Be holy, because I am holy." - 1 Peter 1:16
Holiness is a syncronisation of our natural lives with God's plan for living. It is a practice that brings us closer to God's original plan for how we were meant to live. It connects us to Him and allows love to flow, because practicing holiness will tend to clean away impure motives and darker emotions. Colossians 3 provides a good description of what to do to be holy.
The "what" of holiness is not the goal, and we must remember this. If we concentrate on the actions, we become a form of Godliness but powerless. We rob the very concept of holiness of its power, because we move away from the heart - which is the key to this.
1 Peter begins to tell us why. As Christians, we have to be broken people. We have to be humbled and constantly living in a realistic view of ourselves: we are slaves whose freedom is owed ENTIRELY to Christ, through no work of our own. We live and work on this earth as agents of our God, who is merciful to us, but is also a just judge of men's actions (v13). Through the Holy Spirit we have access to a communion with God that even the angels wish they had (v12). I am a nothing who has been given the highest honor of any being in this universe - a personal and intimate relationship with God. A knowledge of things unseen and unheard.
Out of this comes holiness. We have to respond to a real view of ourselves with a passion for Him. We have to be passionate for His heart and desiring a closer relationship with Him. If we aren't building that relationship, then we're wasting His gift to us - to get to know the one who created the Universe and has all things planned. Do you want to waste His gift? Holy life is birthed out of fear and reverence for His power and gifts, and passion and desire for His mercy and love.
After this, we discover the reason for holiness on this earth (v22). We are called to be holy, because out of holiness comes a purified heart that is ready to love. Our capacity for Godly love will depend directly on our lifestyle. If we choose a lifestyle of holiness, then we will produce a heart that loves. Love is the ultimate goal and result, that ties everything together into one thing that the rest of the world desperately wants and needs.
If we want to love, we must work to purify ourselves of ungodly things, because they will taint our lives and draw us away from a godly heart. This is what we seek, because we know His great love, and we want to share it.