If you study the human body you discover that stress actually serves a valid biological function. It boosts adrenaline so you can run away from that tiger. It withdraws blood from your extremities so you won’t bleed as much if get your arm bitten off. It tends to make you more focused. Etc.
These are all good things. However, stress and its associated biological and psychological functions were only meant to be used in short concentrated bursts. If you’re stressed out all the time, you’re going to get ulcers, heart attacks, cancer, and all kinds of fun stuff. You’re also more susceptible to addictions such as drugs, alcohol, food, and television. Oh, and you’re also more short-tempered and will lash out a family, friend, and co-workers more often.
I’ve noticed that sometimes people make excuses for their stress. “I like stress. It makes me get things done!” As in all things, life is a balance. Stress is good for short bursts. Most of the time you should focus on being happy, not stressed.