We recommend reading Mary's books. Here's a link and exerpt from her blog which explores 'High Challenge Low Threat'.
We are a challenge seeking species. We all know someone, and it might even be us who spends some of their downtime working on a crossword, doing a sudoku, a puzzle or a word search. Why are we spending money and time on things which are essentially 'testing' ourselves? Companies make a small fortune out of the fact that we like 'testing' ourselves. What is that about? Is it because we enjoy the deep satisfaction of struggling with something, thinking hard about it, getting it wrong, going back and correcting it, realising our mistake and then getting it right?