Happiness comes from the inside. The thrill of teaching should be the motivating factor for a teacher, or any one else. That, coupled with the joy of the individuality of each student, are the things that make teaching worthwhile. I view teaching as a calling, if you will. Not everyone has the ability or the desire to teach.
Teaching requires dedication, perseverance, and the ability to deal with failure.
To teach effectively, I feel that a person should be able to care for the each individual for the person that he or she is. If a person does not care about their job, there is no point in the job and it becomes a weighty task. As with any job, the desire to do the job to the best of your ability is a requisite.