From what my T told me years ago you're right. If there's a "reason" it's sadness, grief or whatever. Clinical is a chemical imbalance that needs no environmental cause. Having said that, I THINK that being in a bad situation long enough or experiencing a traumatic event can alter ones chemical balance. Maybe someone will correct me if I'm wrong.
And no, you don't need to go into "why bother" mode! Maybe there's a far better opportunity out there just waiting for you to find it. Don't give up!