It depends what you mean by depression. You can be 'down' but that doesn't necessary mean you are truly depressed - perhaps you are not just happy with your life...
Depression itself has more causes, genetics, neurotransmitters imbalance, environmental factors, social factors, different stressors, any disease that decreases blood supply to the brain... There is probably not a single cause of depression, but rather combination of more factors. Depression also has different intensity, can be persistent, temporal, re-occurring... Personally I believe that personal mind set or believes are also very important factor...
Compare truly depressed person with person who is just 'down', for example:
You are 'down' for some time, nothing is working out no matter what you do, approaches don't work, you start believing that you are a loser and that something must be wrong with you, you can't get ahead in your job... This goes for five years, you are really down, tired, exhausted... but then you suddenly meet a great girl who is interested in you... you would experience surge of enthusiasm, excitement, happiness... you would be in seventh heaven, you would go from 0 to 100 overnight... This excitement may trigger other successes in your life, you simply start approaching things with much different attitude than before, and suddenly things are working out...
A guy who is trully depressed may be in similar situation. He may sleep too much, he may feel worthless, he can't focus on anything and perhaps he even thinks about ending his life... Nothing has purpose, nothing matter, there is no point of doing anything... Say this guy also meets a great girl, and she is interested in him - but he is just not interested at all. There is no excitement, he doesn't want to sleep with that girl, he just sits there, doesn't feel anything, perhaps he is still thinking about ending it all...
There are probably no clear boundaries between true depression and being down, there is lots of overlapping. If you really work hard on something but there are no results for long time, if people are constantly putting you down, if you are persistently telling yourself that you are a useless loser, you are being punished for good work or anything you do... Well, the results will most likely be severe depression, even if there are no other contributing factors.
On the other hand, if you change your mindset, your approach, your believes, if you start believing that it will eventually work out and you just have to "wait it out" - the success is pretty much inevitable. One day it will appear. At the same time, if there are other significant contributing factors, such as genetics, neurotransmitter imbalance, perhaps some underlying disease - you might still be depressed regardless of how much success you experience...