The saddest thoughts are the ones where you remember the past, and note what has changed about it...
Like Hobgoblin. You think of him when he was a petite kitten with two perfect ears, and note that now his left ear droops, never to be perky again, and he will never be petite again....
And think of his brother, Scribble, whose white markings said "Joy," but now has past on, and we hope he still has that "Joy" with him.