Christians allegedly follow the commandments because God told them to. They do what God told them to because of desire to avoid punishment, desire to obtain reward, desire to fulfill their perceived duty, or desire to express their love. They fulfill these desires because it makes them feel good/happy.

Atheists do whatever they do, most of them for the same reason, cut out the idea of it being centered around a personality who effects their happiness.

Harry said he preferred achieving things over happiness, but I can't help thinking that if he had sacrificed his potential, he wouldn't really have been happy about it, no matter how many friends he had.

At the end of the day, happiness drives at least most people, and in theory, all (when they make their decisions through careful consideration, and not just to fulfill some role or habit. As we know, this is rare, and in reality, most people can not trace their decisions' motivation to their happiness or anyone's, or to any other consistent value; so I opine).