Put simply, it may be. However, we are individuals, and thus, we cannot understand exactly what the other person is going through. We can only imagine, and try to empathize. We may have gone through it ourselves even, but there are varying degrees of severity. We can certainly help people out of their depression, however, by exposing them to a positive environment and reassuring them of their abilities, because often depression and low self-esteem go hand in hand. And by doing this constantly, and I emphasize constantly, you can help the person regain his/her confidence and happiness back. Sure, good things might be harder to find in some people, but there is always something good in people, even the smallest thing. Always a silver lining, as my sister would say.
Honestly, I wouldn't be surprised if every single patient who is diagnosed with depression actually have it. This is due to the fact that we probably have a lesser capacity for suffering than people who have to deal with the things that depress us every day. But due to the luxuries that we now have, we don't usually deal with these things every day. Not anymore, at least. I don't claim to be a depression expert, this is just my opinion.