Personally I would say 'have to learn' rather than 'want to learn'. Without some knowledge of English it is very difficult to get a decent job here. That doesn't stop many students thinking that being taught by a foreign teacher is the answer to their prayers and all they have to do is just sit there and you, as the teacher, open a little trap-door at the top of their head - and drop the English language inside.
Many students get a short sharp shock when they realize how long it's going to take them to become proficient in English and many of them get nowhere and give up. Attitude is everything and the Thais fall way short on occasion.
Ian says "It's pretty rare that you get a class who really, truly want to put the time and effort that is required into learning English in order to make a real improvement. Most adults study after a hard days work so they're not at their brightest. If you assign homework expect the vast majority to either forget or just admit that they really couldn't be arsed doing it"