The most obvious reason for becoming a teacher of English as a foreign language is because you may have a desire to help others. As one of the most dominant languages in the world, mastering the English language is often a step to a better career for many ESL students.
Although you may be the person instructing the class, you will also be learning from your students about how to teach. Teaching ESL is often more challenging than other subjects since you may have a class where each student speaks English at a different level, and speaks a different native language than you and each other. These challenges can help you develop your creativity and versatility as an educator.
Although there are many opportunities to teach English as a foreign language in the United States, there is also a high demand for native English speakers to teach ESL in other countries. Many of these positions teaching abroad are to high school and college students who are interested in applying to study in the United States.
Since ESL classes contain students from many different countries, you will have the opportunity to learn just as much about other cultures and languages. Your students may inspire you to become a student of their language just as much as you inspire them to learn English.