Compounds that change are likely to be scattered or split into component parts. Stable unities are not likely to be scattered into parts and thus destroyed. There are two kinds of existence. Forms are invisible and immaterial, particulars are visible and material.
Souls are stable unities, like Forms.
Souls are deathless. Soul is a Form-Bringer of Life to the Body. When a Form-Bringer is confronted by the opposite Form, it retreats or is destroyed. Death is the opposite Form of Life. When the Form-Bringer of Soul is confronted by Death of the Body, it retreats or is destroyed.