I think its just a natural consequence of our mind working on a positive-negative reward system, which I think its critical to any intelligence. Being manically positive is just as detrimental as being chronically depressed.

Its entirely normal to be negative, or to ignore stimuli, or decide not to do things. In some situations, say if you were trapped in a cage your whole life, you'd agree it'd be entirely normal to be depressed. It would make no sense to waste energy running around hitting iron bars that won't break.

In this sense, depression is somewhat of a social construct. We determine someone is depressed because we believe their reaction to the environment to not be normal.