Key to reincarnation is based in duality. 3D is the arena for learning about duality. You are either "in session" here in 3D or "out of session", not currently in 3D, but planning your next lesson, unless you graduate from 3D.

In the event you ask about your multidimensional self (out of session) could take more time planning your next incarnation while waiting in line, or actually do anything you damn well please.

Reincarnation is simply the mechanism for planning, learning and then evaluating your progress in learning the lessons of duality. Which, by the way, you agreed to do.

It's about as simple a system as there is.