Interest is an emotion. I don't think people tend to think of it as such. We say "such and such" is interesting, as if interest is something inherent to that thing. But some people find that thing interesting, others find it boring, some find it infuriating, others saddening. Whatever the case, that thing is sparking an emotion, and our emotions move us. Sadness moves you to change your environment, anger moves you to attack, boredom moves you to find an alternate activity, and interest moves you to explore.
Mike Csikszentmihaly (Chick Sent Me High) did some research into the emotions that people associate with activities. He would plot the activity by the amount of skill it required and the level of challenge it offered. Imagine skill on the horizontal plane and challenge on the vertical plane. Here's what the chart typically looked like:
Frustration - Interest - Engagement
Worry ------------------ Control
Apathy - Boredom - Relaxation
(He actually termed the emotions stimulated by challenging activities anxiety, arousal, and FLOW...).
Learning tends to occurs when the challenge exceeds or matches our skill level, so it might be fair to say that we learn when we feel interested or engaged. This isn't a world shattering insight (Mike's true insight was that engagement increases life satisfaction). However, I think that the role that emotions play in learning is often overlooked.
The idea of "affectivity" has been bandied about in psychology for the last couple decades. People of a certain age tend to have a set-point for their positive and negative affectivity with a level of variance associated with each (take a PANAS questionnaire to determine your own). What this means is that your positive emotions (happiness and the such) tend to stabilize at a specific level, while your negative emotions (stress and fear and whatnot) tend to have their own baseline level. When a stirring event occurs, your emotions usually spike (your level of variance determines how big this spike is) in one or both affectivities before eventually returning to the natural set-point. I point this out to note that the emotions stimulated by challenges and skill will be governed by affectivity. I wonder if each emotion has a set-point rather than just the broad positive and negative categories, and whether that set-point itself is given to a little variability...
A relatively recent trend in psychology has been to actually research why humans feel positive emotions. Negative emotions are easy to figure out. When you feel fear, you run. When you feel anger, you attack. When you feel bored, you find something to do. They focus you. But what does joy cause you to do? What about awe? Barbara Fredrickson has proposed that positive emotions broaden our awareness and allow us to build new behaviors and thought-patterns. Imagine kids laughing while they play together. The emotions they feel encourage them to continue what they are doing, while also helping them to build social skills along with whatever skills the activity promotes. Their positive emotions are causing them to learn.
So learning is encouraged by positive emotions in general and optimized by interest and engagement, but those emotions are controlled by a person's relatively stable affectivity. This suggests that some people are better equipped to optimize their learning by stimulating their positive emotions, since they have a higher set-point or greater variance. As far as I know, neither Fredrickson's nor Csikszentmihaly's research ever actually addressed why people chose to pursue voluntary activities. Maybe the virtues are at the center of all this, pushing people to seek challenging experiences while the vices encourage them to avoid them.
I think that the virtues are tied to emotions: zest breeds joy; gratitude, appreciation; humor, amusement; hope, anticipation; curiosity, interest... Since virtues can be strengthened and developed, it would be possible to increase positive affectivity. Grow a virtue and you increase your ability to feel the related positive emotions. Now virtues are not simple to develop, and there may even be a limit on how much a person can increase a particular virtue (given their nature and time constraints), but the people who do build a virtuous character will be more likely to pursue challenging activities because they are better able to experience the positive emotions that lead them to broaden and build their abilities. So a person with known character strengths that they are actively increasing, may actually also be increasing their ability to learn. We can make better students by encouraging character development... If only it were easy.
Watch Barbara Fredrickson discuss Positive Emotions. Read "Positivity" by Fredrickson and "Flourishing" by Martin Seligman