I like to ride on a bike trainer because it's safe, and it reminds me how much more fun it is to bike on the roads.
I like to bike on the roads because it's good crosstraining, and it reminds me how much I'd rather be running.
I like to run on the roads because it's fast, and because pretty soon I get to run on trails.
But I most of all love to run on the trails, especially through mud, especially in five fingers.
Because, for me, it doesn't get much closer to childish fun than that. :)