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?
I can be either, but for most it’s a symptom and there are many reason, lack of sunlight, lack of social interaction(even for animals), lack of proper nutrition(can cause irateness). But for some it’s not solvable without deep intervention from therapy and drugs.