I think depression is what sets in when a person thinks that they cannot improve there situation by making an effort. It happens to animals, too. Behavior becomes passive. It usually happens after the individual (human or animal) has endured some difficult stressor that it has attempted to alleviate. When it finds that its attempts are in vain, it gives up trying.

I don't think disgust has to be involved, necessarily, but I believe it often can be.

What I said above is called the "learned helplessness" theory of depression. It is possible to come up with different theories.