If a man can write a better book, preach a better sermon, or make a better mousetrap than his neighbor, though he build his house in the woods, the world will make a beaten path to his door.
The point, which can hardly be repeated too often, is that differentiation is not separation. The head and the feet are different, but not separate, and though man is not connected to the universe by exactly the same physical relation as branch to tree or feet to head, he is nonetheless connected - and by physical relations of fascinating complexity.
When we decide to be happy we accept the responsibility to bring happiness to someone else. Some decide that happiness and glee are the same thing, they are not. When we choose happiness we accept the responsibility to lighten the load of someone else and to be a light on the path to another who may be walking in darkness.