Reincarnation occurs because special envoys from the Council of Worlds, loosely termed birthing envoys, gather up disincarnate entities and discuss the entities progress and growth. You may think of this as a school conference if you wish, where the young entity and its guides have a conference with the birthing envoys. The lessons to be learned are formulated, with the most pressing lesson taking priority. In this the entity itself has little input, just as young school children have little input into their curriculum. Many call this karma, where what one did in a past life comes back to haunt one. The entity cannot end an incarnation, except through death or the temporary vacating of the physical body during Out-Of-Body experiences.
Some entities require fewer incarnations at certain points or planes of development than other entities. This variation in the number of incarnations required is based both on the nature of the entity, or soul, and the circumstances of the incarnation. The key is whether the lesson to be learned has been sufficiently learned. In some cases an incarnation will generate more lessons to be learned than it resolved. The entity moves backwards, so to speak.