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?
Keep in mind there are twin studies that have demonstrated the clear genetic association to depression/BP.
This is an incredibly complex question : everything from cocaine rats to gut microbes show there are many facets to the answer, some of which we just don’t know.