What Purpose Happiness

Most people seem to more or less live their life under the assumption that their purpose is to be happy; they might define their own happiness in various ways, but they almost invariably seek it.  We're stuck being alive, whether we like it or not, and so it seems logical for us to think that we ought to try and make our life enjoyable.  But is that really wise?  Is it not conceivable that happiness is merely an opiate, that it prevents us from confronting the harsh reality of our situation, numbs us to the need to make vital changes in our surroundings?

But, then again, what if unhappiness is a sort of mental disease, trying to replicate itself through the promulgation of exactly such theories as this?