Depression is an illness. It is not the same as being temporarily upset when something unpleasant has happened.
Depression attacks the way we think, feel and act, progressively robbing us of the ability to function and live life, unless something is done to stop it in its tracks and restore mood.
There are many physical signs that someone may be depressed (such as weight change, disturbances of sleep and neglect).
Depression can blunt our ability to concentrate and think.
Severe depression can colour every aspect of experience and we may think only of the bad things in life, real and imagined.