LUKE 9:62 But Jesus said to him, "No one, having put his hand to the plow, and looking back, is fit for the kingdom of God."    
Not a single person is fit to serve God and His Kingdom if they look back to their past. You may have noticed that if you are walking forward and you look back you either slow down or stop your pace. Therefore having looked back you lose sight / focus of what is ahead. You may lose your vision for the Kingdom of God which God had given you.  Also putting the hand to the plow means there has been labouring. We are to labour in love for God and His Kingdom.