That's really true. Depression has no face, you really can't tell if someone's suffering from it or not.
There is no immediate solution as causes of it may vary, some may be innate, some may be from traumas of some sort. The best way to help those in need is to just be kind to everyone, because truth be told, no matter how much we wanna help, there will really be times when we might end up not being there for others all the time.