From an evolutionary standpoint...?

I can only think of a physiological reason - the release of endorphins which are feel good chemicals, and laughter increases the production of serotonin which elevates mood. Both these chemicals can help reduce stress and illness such as depression, so from an evolutionary point of view, the reduction of stress would suppress the ill effects caused by the stress hormone cortisol, and therefore increase life expectancy, and that would be survival of the fittest, to find a coping mechanism to deal with the stressors in life.