I like the concept of ‘flow’, as described by Kellee Santiago, TED Fellow, game designer and founder of thatcgamecompany:

A fundamental aspect of game design is creating “flow.” A state of flow is achieving the right balance between challenge and experience. If we’re presenting the player with something that’s too difficult for them while they’re playing it, they’re going to be really frustrated. But on the flip side, if it’s too easy, and their skill level is very high, it’s going to be boring. The sweet spot is called the state of flow.

The term comes from the psychologist Mihály Csíkszentmihályi and his observations of people in the 1970s. He studied how people achieve happiness and how we define it. He came up with the word “flow” because most people describe that feeling as being carried along in the water. So it not only is a great tenet of game design, but it really applies to our whole life, and life design.

More here.