Yes, nothing works as well as saying 'Cheer up' to a person with Depression. </sarcasm>
I can only assume you've not experienced the black dog up close, or had someone close to you suffer. For those of us who live with it, it is a truly terrifying thing, that can cripple you to the extent that mere breathing seems like effort.
I'm not being a dick to you by the way, just highlighting that cavalier responses such as yours is why Depression has the taboo it does.