Earlier quoted context omitted.
Why would you not support your friends through depression?
Because supporting depressed people is depressing and often futile. Mental illnesses can be transmissible that way. Also, supporting a depressed person needs lots of time and effort, and while they are depressed they give nothing back except bad vibes.
edit: I just found this meme: https://i.imgur.com/WCzNW77.png and I decided to link it here. People think friendship is something that is built over years because it takes years in order to have finally made it through enough hardship and shared struggle so calling each other friends is justified. This is the literal meaning of having a friend. You know they'll help you even you got nothing to give and without expecting anything in return from them.