I am thinking there are a lot of components...acuity of vision and hearing, physically orientating towards the activity, visual and auditory processing, comprehension and appropriateness of the activity you are supposed to be attending to, interest, stamina, the basic needs having been met (food, drink, sleep, shelter, attention, exercise, etc.) and on and on.

...reading is pleasure, not just something teachers make you do in school.~B. Cleary