[Jolly]

I said it was a logical way of thinking about depression, not what the logical answer to depression is. Its not logical that depression exists to some, as there is no real need for depression in the world. ( Hence making it illogical that it should exist in the first place )

Not having a reason to exist doesn't equal with it not being able to exist.
Life by itself has no reason to exist, planets and stars can be born, without them being flourished with life.

Also if you look at depression from a genetic point of view, it could fall into the category of natural selection, those with weak genes and the inability to adapt die out.