Sometimes I come across quotes and ideas by people that resonate so intimately I can't help myself but share them:
"You can’t motivate people. The best you can hope for is to inspire them with your actions. People who think they can use behavioral “science” or management techniques have not spent enough time on the receiving end of either."
— Leo Babauta (Zen Habits)
The science of motivation, and of fun, is quite clear on the subject. It's very easy to motivate someone who wants to do something. It's near impossible to motivate someone who doesn't want to. Which is why leisure and luxury sells well, while real fitness and learning looks stuffy and boring.
This is a moral question for designers. Because the easy design, the quick sell, will be motivating. It will also, quite probably, be the wrong thing to do.