Augustine had four movements of the soul: desire, fear, joy, and sorrow [cupiat, metuat, laetetur, argrescat]. These were interrelated and hinged on humanity desires. Thus will was of supreme important for Augustine: if the will is wrong, these motions will be wrong; if the will is blameless, the motions will be blameless. Will was at the center of his ethics.
Returning to these movements of the soul and their connection to desire, Augustine posited that joy and desire were bound in consent to the things we wished for. Fear and sadness were seated in aversion to the things we do not desire. When we seek the things we wish, it is desire. When we have the things we wish, it is joy. When we avoid the things we do not wish, it is fear. When we have the things we do not wish, it is sadness. All hinges on our will, our wants, our wishes. Thus, the different affections are bound to our will.
When we live to God, we love good and hate evil; our will is controlled by our devotion. The love, joy, fear, and sadness are right when controlled by a right will; they are evil when not so controlled. Scripture approves of all these affections, provided the will is right, and a right will is a one drawn to God. Even so, they are peculiar to this life, and stem from our weakness as men. Augustine still stresses it good for us to live with them; impassibility may be desirable, but is not attainable in this life; those who live in supposed apathy lose their humanity. In the afterlife, Augustine seems to think, we will have rightly ordered joy and love, but not fear or sadness (see City of God 14).