Because depression is so thematically powerful and so dark, when it's very severe, it can make people feel not only as if they've lost a loving connection, but as if the whole world is devoid of love. So if we wonder how somebody could take 149 people with him when he commits suicide, one answer can be that depression, when it's most severe, can make people feel that life is completely without value, not just for them but for anyone.
Keith Ablow