Arjuna: Who are the creatures dwelling in the upside-down tree?
Krishna: The living creatures in this living world are certainly eternal fragments of me. Situating themselves within material nature, they draw unto themselves six sensual capacities, headed by the mind.
Thus the soul develops and dissolves its bodies. Like the wind carrying a scent, the soul carries the subtle elements of one body into the next.
Arjuna: Why does the soul change bodies?
Krishna: The soul changes bodies because it wishes to enjoy sense objects in various ways. Each body offers a different aural, visual, tactile, gustatory (taste), and olfactory (smell) array.
Sometimes staying in a body, sometimes giving it up, sometimes enjoying the wealth of the three qualities; those with informed vision can see what is really going on, but great fools cannot recognize it, even when shown.
Serious spiritualists can also see the soul doing all these things, but those who are serious to avoid the soul are basically unconscious, and thus blind to it.