It's considered an illness because it interferes with a person's ability to live a normal life, hold down a job, raise a family and enjoy life. Depression is a horrible thing to live with and I wouldn't wish it on anyone. I am not a fan of Big Pharma and I believe mental illness has been exploited for profit. But don't go saying that depression isn't an illness. I can assure you that it is because I've lived with it for 40 years. When everything in your life is going great and you have everything to be thankful for yet you're still sad for no apparent reason, one cannot say "oh, nothing is wrong with me, it's all in my head," because that solves absolutely nothing.