What makes racing fun?

Me and Renzo are working on a paper on how to automatically create fun racing tracks. And not only that, but racing tracks that are fun just for you - which means that we must first model how you drive with a neural network, and then use this model of your driving to create the tracks.

Grand plans, but will it work? Well, we'll see soon. In the meantime, here's a question for all of you: what exactly is it that makes a racing game fun? And what is it that makes a particular track/circuit in racing game fun?

Of course, in the end we want some quantitative measure we can put into our code, but the quantitative part is really up to us. What we're looking for right now is suggestions, ideas. Any sort. What makes racing fun? Please leave your comment below!