This may sound like a easy question, but the better you understand your game, the better you can improve your it.
Answers like ‘just because it’s hard’, or ‘because it has cool weapons’ only skim the surface. In this case, you’ll get closer if you say it’s because players like the challenge in your game. Better said, they like to feel competent, effective in what they are doing. Both in our work lives and our leisure we are intrinsically motivated to seek out opportunities to experience competence and the satisfaction that accompanies it. So naturally, this also leads to games and the challenges they offer.
According to research of Richard Ryan we have three innate psychological needs: competence, autonomy and relatedness. When these are satisfied, we are motivated, productive and happy. To see how this applies to games, defnitely read the article Rethinking Carrots on Gamasutra.
Earlier I gave the tip find your intrinsic motivation, you can find the answer somewhere in these 3 needs. And in the same way, you can better understand the intrinsic motivation of you players when they’re playing your game. Find which ones work best for your game and your players.
The first prototype for Circles had something fun about it, but it took some time find exactly what that was. I thought an aspect of it was in its puzzle challenge or its navigational challenge. I tried out tricky levels with narrow spaces, switches that would open areas up and in general more complicated levels. But it seemed focusing on more challenge and satisfying a need for competence did not work out well for the game. Instead, I switched and tried to focus more on letting the players experiment and discover how circles behave. This meant simpler levels consisting of just one or a few circles and made making it clear to show off their behavior and interactions. This focus on autonomy worked a lot better for the game and was a lot more engaging for players.