Anyone noticed, Studying by your will is more effective than studying because you have to?

Example. For me, I would love to study books and videos, of knowledgeable things and scientific research etc. However when it comes to school, even if it is a relatively interesting subject. The desire to learn about it decreses. As a result, it causes me to study the same thing a couple of times to understand. On the other hand if it were something I didn't have to study or learn I could read a whole book about that subject once or twice and bingo. I'd know everything coz of my interest in the subject. I guess it is because when it comes to school, we have to learn it. I think if everyone was enthuisatic to learn and interested, everyone would learn upto 10x better. Thoughts? Any idea what is the psycology behind this?