Happy families are all alike; every unhappy family is unhappy in it's own way.
  - opening line of Anna Karenina by Leo Tolstoy

Interesting thought.  Does it apply to individuals in the same way?  Maybe what makes each person an individual is his or her own unhappiness and in our happiness we all become alike.  Does it mean happiness is universal?  Does that explain why depression is isolating, but people look to gather during the happiest moments?