[Tichy]

This german article gives some good reasons: http://www.spiegel.de/netzwelt/web/0,1518,591783,00.html (in german, unfortunately). Maybe some people really are cruel, but there are other reasons (as with the shoutings of "jump already" if somebody jumps off the roof). People resent being manipulated, which a suicide does (just look at the pushy comments here, about what justin.tv HAS to do now). And by saying "so what" they make the suicide seem meaningless, which might help in some cases to prevent it - if the suicide candidate wants the attention but doesn't get it.