For someone who hasn't experienced severe depression, it can be difficult to understand why it can be so debilitating.  What they don't understand is that depression isn't an extended case of sadness.  Depression is a real health problem that interferes with every aspect of a person's life.
 
Depression is a disease that effects 5% of the global population.  It is caused by an excess of monoamine oxidase in the brain.  In a healthy individual these enzymes would break down excess mood-related chemicals such as serotonin and melatonin, leaving the necessary amount needed in order to react to daily stimuli without too much difficulty.  In a depressed individual, MAO-A is produced in excess causing essential chemicals to be destroyed.  Without these neurotransmitters a person can have difficulty functioning.

A successful individual is expected to be productive occupationally, socially, and personally.  Not only are people expected to work efficiently at a job, but to maintain social relationships, and to fulfill family obligations.  All of these many interactions are fueled by aforementioned neurotransmitters that tell the mind how to feel in certain situations.  They help maintain feelings of stress and anger and keep the individual working towards his/her goals.  They are what motivate us even in the face of defeat and keep us hoping for a brighter future.  Depression sucks all motivation out of an individual making everyday tasks and obligations impossible to complete.
Depression is an indiscriminate disease.  It could attack any healthy person, destroying their life if it goes untreated.  Some people suffer from acute depression that leaves after a time, whereas some people suffer from chronic depression that stays with them until they die.  Some people will never suffer from depression.  You can't single out people that are more likely to "catch" depression.  Both depressed and healthy people have goals in life.  They exist on all spectrums under all conditions.  The only thing that differs from the depressed and the healthy are a few neurotransmitters, or lack thereof.  With effective treatment, you might not even know it's there.

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F*ck off.