You just give up on living. It's devastatingly sad. I don't think it is a single motive, it's the piling up of tragedies, desperation, lost of hope. You lose connection to the world, to people, everything becomes meaningless and inanimate. If you already feel worthless and a failure, one single disappointment can throw you down the edge. I cannot judge who choose this path, everyone considers it at some point in life, I can only pray that their suffering ended, and morn for those whose life were taken without cause.