Sadness has always been very much part of the anime and manga world, at least in the "classic" works (up to the 1980s). I've always wondered if this was a side-effect of the Hiroshima and Nagasaki tragedies. The way that Japan seems to have taken it all in stride and reconstructed quickly after the war is proof of that people's proud and industrious spirit, but the damage can't have been without consequences on that nation's psyche. The works of all the great masters (Leiji Matsumoto and Hayao Miyazaki, notably) are always permeated with a lot of sentimentality, but also sadness. Death is very often an integral part of their works, as are courage, pride, independence of spirit and friendship.