[A man] cannot help envying [animals] their happiness — what they have, a life neither bored nor painful, is precisely what he wants, yet cannot have because he refuses to be like an animal. A human being may well ask an animal: ‘Why do you not speak to me of your happiness but only stand and gaze at me?’ The animal would like to answer: ‘The reason is I always forget what I was going to say’ — but then he forgot this answer too, and stayed silent: so that the human being was left wondering.
In the case of the smallest or of the greatest happiness it is always the same thing that makes happiness happiness: the ability to forget or, expressed in more scholarly fashion, the capacity to feel unhistorically during its duration…. He who cannot sink down on the threshold of the moment and forget all the past… without growing dizzy or afraid will never know what happiness is — worse he will never do anything to make others happy…. Imagine a man who did not possess the power of forgetting at all and who was thus condemned to see everywhere a state of becoming: such a man would no longer believe in his own being, would no longer believe in himself, would see everything flowing asunder in moving points and would lose himself in this stream of becoming… he would in the end hardly dare to raise a finger.
Nietzsche on history and happiness (and flat ontology)