People who have had no opportunity to hear of God and of Christ's sacrifice for our freedom, likely will return to earth as often as necessary to learn the reason for their incarnations. Those who have received enough information to be convinced of God's existence, can use each incarnation to improve their character and their faith and trust in God and in Christ.
If after having received the correct information, they decide to go their own way in order to follow the ways of the world, how much more difficult will it be to have a change of heart and to begin to trust God once more. To abandon God and Christ for worldly comforts will require more incarnations on this earth, rather than an opportunity to continue their progression in a higher realm with fewer problems and more happiness.
A Story of real reincarnation http://godsgrandplan.org/page15
Swiss spirit messages of incarnations, spiritual progression. http://www.glzh.ch/index.php/en/spiritual-teachings/spiritual-world-online