There's a beautiful poem by Giovanni Pascoli, "Aléxandros" in which Alexander the Great gets to the Indus river with his army after having conquered half of Asia; he knows it's the end of his conquest and thinks - it was better when I was dreaming of it, I was happier when more challenges, road, destiny where ahead than behind.
James Clear talks about this in his book Atomic Habits. Basically people are happier during the _process_ of trying to accomplish some goal than when they actually accomplish that goal.
