As Thailand attracts more international visitors, and as the country's exports grow, English has become more important to everyday Thais. In addition to the huge number of Thais working in tourism and other industries that require the use of English, there is also a large number of Thai students studying abroad, predominantly in the USA and the UK, and to a lesser extent in Australia, New Zealand and Canada. There is demand for native English speakers to teach in Thailand and as such, teaching has become a popular, perhaps even the most popular, choice of job for Westerners who wish to live in Thailand.
For foreigners who choose Thailand as their new home, English teaching provides the opportunity to get work. Becoming a teacher of English is one of the few jobs that most Westerners can do and actually carry out legally in Thailand. There are always loads of TEFL jobs in Thailand!