The motivation for each student is likely to be different. I can remember one young man who was completely uninterested in learning English, didn't wish to be on the course .... After a few days he met a young lady from another country and the only way they could communicate was English. Instant motivation, 100% in class and lots of practice outside!!!
There is a place for motivation but as my old DoS said - You can lead a horse to water, but you can't make it drink.