Attean makes it quite clear that he dislikes reading lessons and does not want to go to Matt's place to learn to "read the white man's signs", yet he keeps coming back, day after day. Matt speculates at one point that Attean returns because he wants to hear more of the story of Robinson Crusoe, from which Matt has been reading carefully chosen excerpts aloud. I imagine that this could be true, but I think there is a deeper reason for Attean's persistence as well. After all, when Matt read about Friday bowing to Crusoe, his white master, Attean was furious, and clearly showed he wanted to hear no more about an individual being enslaved to a white man, yet he still returned the next day.
I think the thing that keeps Attean coming back for the lessons has to do with a matter of honor. Attean's grandfather Saknis has made a "treaty" with Matt - in exchange for food, which he obviously needs, Matt will teach Attean to read. Although Attean makes it clear from the beginning that he does not like this arrangement at all, Saknis is unaffected by his voiced rebellion, and has no doubt that his grandson will obey him. In the Indian culture, honor and respect are essential. When all is said and done, Attean would not think of defying his grandfather, and to not keep his part in the treaty his grandfather has made would be dishonorable and even unthinkable.