Frustration engages

Now in my way to the temples of Angkor in Cambodia, I’m sitting in a minivan full of people, and there’s a Chinese girl playing a videogame in her smartphone. And she’s frustrated.
She’s trying to pass the third level in the game and can’t.
It’s been at least half an hour and her engagement’s level with the game has increased.
Finally she passes it, and keeps playing.
The game is not easy, but it’s not complicated either. That’s the point. In that gap is where the real engagement resides.
Too much frustration will make it overwhelming. Not frustration at all will make it boring.
Adding a little frustration is good to increase the engagement. That’s what keeps someone playing the game.