According to Iamblichus, souls incarnate for one of three reasons. Most people are dragged into incarnation by their daimons. The souls, or their pneumatic vehicles, of such people, what would be termed the "mass herd of humanity," are so far off their path they must incarnate for further purification. The second kind of soul, what might correspond to the philosopher or theurgist, still incarnates for the purpose of purification, but there is also something of a willingness of the soul in its incarnation. It knows it needs more purification and so acquiesces to its fate. The third kind of soul is fully purified, associated with what I call the theurgic sage. Such must incarnate because that is the nature of human souls, but does so with a complete coordination of the soul with its telos. This soul is above Fate, which affects only the sub-lunar realm, and basks in divine Providence. Iamblichus says such souls incarnate in such a way that they, unlike the other kinds of souls, never identify themselves with their bodies. The body remains subject to fate, as this is part of nature, but this soul, because it has not confused itself with its body, is not.
That doesn't answer the question. We're getting there. Honest.
*What constitutes purity and impurity is a whole different discussion.