Interest and Learning:

"If interest doesn't enhance learning because of heightened attention, then how does interest affect learning? Research suggests that depth of processing might be responsible. ... Perhaps interest leads people to process text more deeply. In turn, deeper processing leads to better comprehension and recall of the text. Many experiments support this position (see Schiefele, 1999)." 

"Interest appears to promote learning through several mechanisms. Intuition to the contrary, interest doesn't seem to increase attention. Early experiments found that interest increased attention to text (Anderson, 1982; Asher, 1980); recent experiments found that interest reduced attention (McDaniel et al., 2000; Shirey & Reynolds, 1988). A second mechanism, depth of processing, has received stronger support. The interested reader approaches and processes text differently from the uninterested reader. Interest promotes focusing on the text's meaning and building a propositional representation, whereas boredom promotes focusing on the text's superficial aspects (Schiefele, 1999, 2001)."

Paul Silvia, Exploring the Psychology of Interest