How is depression different from sadness?
Sadness follows losses or disappointments, and is a normal part of any fully lived life. Healthy people eventually rebound, and apart from periods of grief or mourning rarely cease functioning in daily activities.
Moreover, healthy people do not tend to blame themselves for the normal losses of life, whereas depressed people often feel undue guilt or self-criticism.
Finally, individuals with depression often experience marked sadness in the absence of any cause, whereas healthy individuals have moods that relate more to life experiences, and moods that are in proportion to the intensity of those experiences.