There is no law of nature guaranteeing that mankind should be happy.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“But even if the heart said the same thing to all men, that could afford no evidence for the existence of anything outside our own emotions. However ardently I, or all mankind, may desire something, however necessary it may be to human happiness, that is no ground for supposing this something to exist. There is no law of nature guaranteeing that mankind should be happy. Everybody can see that this is true of our life here on earth, but by a curious twist our very sufferings in this life are made into an argument for a better life hereafter. We should not employ such an argument in any other connection.” source