I'm sure a proper psychologist could dig into the precise reasons why some people turn out happier than others, but something I've learned from watching my students is that in the same unfortunate situation, a guy can choose to be happy and optimistic, or he can choose to be a negative Ned. Negativity, the seemingly natural choice in tough situations, almost always makes the situation worse. Choosing happy often makes the situation better. Happiness makes friends who rally to the side of their optimistic colleague. Habitual happiness, though not always the natural choice, often wills success into existence. So often students with crappy backgrounds who choose happiness anyway end up with B's and C's while their peers from similar backgrounds fight to get D's and F's.
Most of us can appreciate the positive energy and raw strength demonstrated by our happy colleagues. A happy, optimistic worker is a powerful force, and is an unsung hero of the workplace. They turn the mood, make people believe that difficult situations can be won; or at least help us not overly dramatize the trivial emergencies! Their energy extends into peoples lives outside of work as well... negating some of the pessimistic energy that tends to be rampant in the workplace.
After battling-away at breaking plateaus and attempting to tackle goals that seemed audacious to me (like running 50 miles!), I've begun to realize that performance begins to grind to a halt unless I am willing to learn to be habitually optimistic and believe that nagging injuries CAN heal, that weak fingers CAN get stronger, and that a fearful mind CAN become brave. Optimism and happiness are not only a weapon for being a better colleague and teacher at work, but they are also key to reaching new levels of athletic performance, and inspiring myself with increasingly improbable feats.
Things will always go wrong. There will always be factors that make my life at work and as an athlete difficult and not fun. I can't change the challenges, but I CAN change how I respond to them. As I've seen while observing students, choosing to be negative in the face of a challenge will almost certainly not help; but choosing to be positive very well may.