I once did an observation of an adjunct English professor. Her lesson was fine: clear, logically organized, multi-modal. The students remained on task. No fires broke out. I couldn't help but feel, though, that something was missing. Then I realized what it was: She didn't look like she was having any fun.
A great deal of the teaching profession, let's face it, sucks. Lesson-planning is tedious. Faculty meetings are endless. Grading is at least the fifth circle of Hell--the seventh or eighth if you're grading essays. But what makes teaching bearable is the hefty paychecks. I kid, of course.
No, the only part of teaching that provides any sort of consistent joy is the time spent actually teaching. Not always, of course. Some classes can suck the life out of even the most Panglossian soul. On the whole, though, the classroom is where a teacher gets to shine, to show off, to revel in the attention, to sublimate the urges that would otherwise drive him to rank exhibitionism--or acting! Frankly, any successful teacher--excuse the expression--gets off on teaching. And any teacher who doesn't should seriously consider a different profession.
Obviously, not all jobs are fun, and "fun" may not be the main consideration in one's job hunt. Teaching, however, is disproportionately difficult given the salary one can expect to receive. People can make similar money doing jobs that are significantly less draining--salt mining, for instance. If you find yourself collapsing at the end of the day, that's normal. But if you find yourself bored while you're actually in class, get out as soon as possible.