Turning negativity around can be as simple as getting somebody to laugh at a stupid joke or reminding them of all the happiness in their lives. But when somebody is depressed, they are immovable. Your jokes aren't funny to them. Their happy memories are illusory and fleeting. Reminding them of good times only makes then harp further on bad times.
Depression is some seriously scary shit. It is a mental block, or perhaps a filter, and it is certainly not how the mind should operate. And because of all of this, it is something that should be explored, both from a person experiencing it, and from an outsider looking in and seeing it happen to somebody else.