For a lot of people it is without a known source or trigger or point of orgin that depression will sit in.  They may be fine one moment and then the next a dark cloud suffocates their heart and  venomus thoughts stab into their minds.  And all they can do is shut down, close themselves off, climb in to a dark hole and then mentally and emotionally abuse themselves.  Afraid to say anything because we will call them crazy or too sensitive or worse completely ignore the problem and say something like "man up wuss"  or "you're such a pretty girl, smile don't be sad"... We send alcoholics and junkies to rehab with love and well wishes but treat depression like its nothing more than a bad day out of life, here's a lollipop stop your crying.  People are broken inside and lost and feel alone and very often it's because somewhere in their life they just cared and loved too much... And we repay their kindness and generosity with apathy and ignorance and half hearted sighs as we roll our eyes.  No one stopping to imagine the unimaginable burden and darkness that is going on inside their hearts and souls.  No one able to bare to try and feel for a moment how bad some one has to hurt where death is the better option than life... Let me repeat that... that death is the better option than life ...that the pain and hurt and thoughts have become so bad that they feel death is the better option.  And too many caught in that moment, feel they have no one to turn to, no one willing to accept and belive them that they are depressed and that they just need at least one person to be there with them, that no one is willing to belive they hurt that bad... and then.... BANG ... their gone.  And then for a split second we can all hear the silent monster and see the invisible disease, a second to late to help and a second many of us will wash clean from our eyes.  Because its just to horrific to remember and we're afraid if we open ourselves up to the reality of depression we ourselves will end up another one of its victims.  
Depression is real and it is real scary and those suffering need our acceptance and acknowledgement that they are suffering a horrific disease most of us couldn't cope with.  When we know someone who has it so bad they can't get out of their hole, we need to climb down that hole and sit down in the dark with them.  If they want silence we give them silence, if they want to talk we listen, if they want to cry we cry with them.  We let them know we belive them, give them our compassion and love and empathy.  Don't just be there for them, be there with them.  If we just give them a ladder out, they might come up but then when were not there with them, they might just fall back in.  You really have to let them know, I'm here with you, in or out of this hole, I'm with you, you're not alone.  It's ok to be depressed, its ok to be you.  Maybe you'll get better, maybe you won't, either way I'm here with you.