At some point in your career, you will experience burnout. Regardless of how much you like your job there will come a time when you just don't feel like doing it anymore. If you could choose between being sick enough to stay home (and not just lying about being sick) and going to work, you would actually choose to be sick. It would be far less aversive than facing your boss, your co-workers, your clients, and your desk.
What exactly is burnout? It is defined in Merriam-Webster's Collegiate Dictionary as "exhaustion of physical or emotional strength or motivation usually as a result of prolonged stress or frustration." Who's stressed and frustrated? Many people. Mass layoffs are making workers very nervous. Many are afraid of losing their jobs and are therefore working harder and longer hours to prove their worth. Survivors of layoffs have to work harder to fill the gaps left by their departed colleagues.
Then there are those who work hard and don't receive the gratitude they feel they deserve from their bosses. They go to work everyday, work hard, and don't feel they are rewarded properly. Raises aren't forthcoming, and promotions aren't either. People who seem to work less, but have more political clout, seem to do better. Sounds frustrating to me.
Being in the wrong career is also very stressful and can be frustrating. Many people are in the wrong career. They either tire of a career they once liked or they chose poorly in the first place. Others are in the right career but in the wrong job. Either way a change may be in order. It may involve a career change or simply a change in where you work. So, as you can see, there are many factors that cause people stress and frustration with their jobs. I'm sure you can name some yourself.
Burnout doesn't happen only to those who are stressed or frustrated, though. Notice the definition says burnout usually happens as a result of stress and frustration. I don't think the stress that causes one to experience job burnout has to be terribly obvious. Work may be going along smoothly. There are no apparent problems — no issues to resolve. You get along well with your boss, co-workers, and clients. Then suddenly one day you feel a little knot in your stomach when you think about going to work. Or, you can't come up with any fresh ideas. You let your inbox fill up. You cringe when your phone rings. You just can't figure it out. Yesterday you loved your job and today you hate it. What could have caused this to happen?
Many of us work long hours because we actually like our jobs. We have work that needs to get done, and we choose to spend ten hours a day doing it. Then one day we realize that many months have passed since we had a vacation, a full weekend off, or even a relaxing evening at home. There's an old saying that goes "On their death bed, no one ever said, 'I wish I had spent more time in the office.'" As an aside, the man who coined the term "burnout" was a psychologist named Herbert Freudenthal. Dr. Freudenthal, himself, had a reputation for working extremely long hours but did not experience burnout.