That it's not required is the fun part, and really is necessary for it to be a good idea at all. See GK Chesterton:
"Drink because you are happy, but never because you are miserable. Never drink when you are wretched without it, or you will be like the grey-faced gin-drinker in the slum; but drink when you would be happy without it, and you will be like the laughing peasant of Italy. Never drink because you need it, for this is rational drinking, and the way to death and hell. But drink because you do not need it, for this is irrational drinking, and the ancient health of the world."
Drinking during celebrations are some of my favorite memories. Its like we're all already happy and high on good news, think weddings or big sales news or getting a big project completed. And then we drink because the worries of the world are behind us, now the happiness is uninhibited and jovial. I like to think those moments add time to your life.
"Drink because you are happy, but never because you are miserable. Never drink when you are wretched without it, or you will be like the grey-faced gin-drinker in the slum; but drink when you would be happy without it, and you will be like the laughing peasant of Italy. Never drink because you need it, for this is rational drinking, and the way to death and hell. But drink because you do not need it, for this is irrational drinking, and the ancient health of the world."