Infrequency
Which crowd seems happier: that of the Lakers when they score a basket or that of Manchester United when they score a goal? To me (and to anyone who knows anything about soccer), it is definitely the latter. But why? What causes such an uproarious riot when a soccer team scores that seems to make basketball fans seem like they're sleeping turtles? It's not a mystery that the key to soccer fans' happiness is the infrequency of the scoring: basketball teams frequently have scores above 100 whereas soccer teams rarely top 5.
This same principle can be applied to many aspects of life: the rarer something is, the happier it makes us (just ask any man who's pissed off his wife or girlfriend: diamonds and gold always do the trick). For instance, when I went to a relatively poor neighborhood in a third world country last Spring, I gave a child something so simple: a hug. Yet this simple act made the child happier than any hug had ever made me. Happiness, you see, varies depending on the person. One who finds positive events familiar will be less pleased by an extraordinary event than one who rarely encounters such events.
That being said, it also seems to be true that infrequent events can lead to greater anger. For instance, if a girl is use to getting anything she asks her parents for, an unexpected no can lead to considerable animosity. However, if a different girl has become accustomed to rejection, the anger will be much less. Again, this is evidence of the importance of the person on how happy something will make them. The answer (no) is the same, yet the behavioral response to the answer differs depending on how the child was brought up.
The lesson, then, is that we must learn to not take positive events for granted. With each joy, we must express our happiness and recognize that we are lucky people. Think about all the things that have made your day just a little bit better and hold those things dear, for they may or may not show up again. In particular, if there's a person in your life who makes you incredibly happy, let them know. Take a lesson from those who may not be so habituated to having good people around: when they do come around, act as if they're the greatest thing that's ever happened to you.