One of my luckiest instincts lay in being able to tell when I was
happy--at the time, not afterwards. Most people don't realize until
long afterwards that they have passed through a period of happiness.
Their enjoyment takes the form of reminiscence, and it is always
tinged with regret that they had not known at the time how happy they
were. But I knew, and my memory (of bad times too) was detailed and
intense.
-- Paul Theroux. My Secret History (225)