There’s 200+ different viruses that cause the common cold. When you get a sore throat or runny nose you don’t consider those to be the disease, you know there is an underlying cause.
Do you think Depression is similar to that? Do you think it is a symptom of some other disease or diseases?
Its a disease, it has its own symptoms. If two people get a virus they will both catch a cold. If two people lost their job one of them might try to commit suicide while other one might only gets sad.