Flow the Psychology of Optimal Experience by Mihaly Csikszentmihalyi

The most important/influential part of Flow the Psychology of Optimal Experience came early on in the form of a graph illustrating the concept of flow channels (below). To put it simply, a person will stay engaged with an activity as long as the person faces the appropriate level of challenge to match their skill level. If someone's skill in a task outgrows the level of challenge, then they will quickly become board and lose interest. If someone is up against a challenge that exceeds their skill level, they will experience anxiety and frustration. Difficulty should increase to match the person's skill level as it grows.