If a soul chooses to reincarnate, it will will have to form an entirely different personality than it had in the last life. Aside from the decisions made in its life plan concerning its birth circumstances and its ultimate life goals, there are all the usual physical and sociological factors that the soul encounters while living in an entirely new body and in an entirely new environment. All of these new factors combined with the soul’s ongoing use of its free will can affect the new personality in an infinite variety of ways, especially during the formative years while the child grows up.
The question here is what happens to the original personality after the soul reincarnates? Does it just cease to exist? Does it continue as a separate entity in an afterlife while the soul that created it spawns a new personality in a new body? Does the old personality become an ingrained part of the original soul which then goes on to reincarnate using traits of the old personality to form its new personality? Or do souls not reincarnate at all which would mean that all spiritual advanced evolution takes place in the afterlife?