You see God did not create man as a robot where we would just respond to everything he has said, or commanded, or required that we should do.  Man is a free agent, and he has the freedom of choice to reject God or His Son and/or this free gift of eternal life offered through Him.  But even though we have the freedom to choose as we wish, we also will be held accountable for our choices in life, and man is accountable to God.