for some people it's a phase in their life, e.g. during high school / college...
and most of the time it involves relationships / death / hopelessness...
best way to handle it is through talking, encouragement, convey hope...
need to avoid self pity... get them involved in activities outside the house, into the community...
volunteer in orphanages... homeless shelter... senior nursing homes... etc...
be thankful for the good things... make the best of what life throws at you...
there is a light at the end of the tunnel...