I think perhaps you do not believe that depression is a mental illness. Perhaps you also believe that drugs and hormones can not affect mood, that heroin and alcohol have no affect on the body. Or perhaps you believe that there are absolutely no circumstances in which a human can grow to adulthood without exactly normal sensitivity to mood affecting drugs and hormones. Like how we all evolve slightly different nose shapes and patterns of hair growth, but all the organs in the whole body act with perfect regularity with drugs with no variation at all.
I don't know, as someone whose 8 year relationship ended with a partner who never accepted depression wasn't something I can control. To me its like not believing that lung cancer exists or something. We have evidence, diagnosis and treatment for it. Depression is hardly a made-up condition.