One way I would think of depression is, it's a condition that is triggered by a case where it seems like you fundamentally as an identity don't fit the current environment, and there doesn't seem to be a way to change the environment, or yourself to adapt to that environment. Nothing is going to make you happy, because there is something fundamentally wrong. And in many cases it won't be obvious that something is fundamentally wrong. It could be that you are genetically not wired to adapt to modern environment, which was never there. For example you are forced to go to school when it doesn't fit you, later in life forced to work at something that is not according to your talent or passion.

I wouldn't call it like a reaction to fear.