http://www.depression-guide.com/symp...depression.htm
A big part of it is why your depressed. For some people its genetic based and causes the chemical changes in brain for no good reason. For others it can be in response to major life changing events or a series of events, build up of stress etc. And for some it can be a mix of both.
At least if you understand why you get depressed you can learn to deal with it easier.
Remember for most people a depressive episode is just that and eventually make a full recovery. But for some it can be a ongoing nightmare of recurring episodes throughout thier lives.
Apparently i get depressive episodes quite often but i try to not let things get too bad these days. But its real hard when theres just no energy there, no nothing, etc to just function daily.