When you have high motivation level to studying, you get a risk of exhaustion. For example, when if you study Japanese, you will step for...
- I wanna finish to learning Language soon as in today!
- I'm reading Japanese book but i don't feel improving.
- How much efforts and time are needed to learn Japanese?
- I think it's so long long way, i wanna surrender studying.
You see high motivation will always not be effective for studying.
High motivation has a risk for exhaustion, but has an advantage make learning faster. Low motivation has a risk for slow learning, and has an advantage you will be not tired.
I think appropriate motivation is necessary to long type studying. Best motivation level for you is does not make you tired, and does not make your studying slower.