Basically, the problem involves the will.
Man is a free moral agent.
When God created us, He gave us a mind and will of our own. God would be breaking His own spiritual laws if He forced us to do His bidding. At the time of conversion our will is yielded to the will of God. They no longer seek to do the will of God, but for various reasons have chosen to go their own way. They have decided to work out their own plan and purpose for life.