I feel like the number one motivating reason is having a significant other who has a different native language. This also makes it a lot easier to make progress because you basically have your own tutor who is also motivated to patiently listen to your struggles.
There's a Spanish saying that goes something like: the best way to learn a language is in the bed or the crib.
Nothing is quite as motivating to learn a language than dating someone (or, more likely, trying to) who doesn't speak English very well or is willing to refuse to speak it.
