It all about interest in combination with comfort and discomfort. Success or reward brings you in a comfort zone. At least, thats my experience. When you start with interest in a zone with discomfort, suffering, thinking hard about new things, studying, social issues, etc, …… it´s hard to persist the same level of interest without a break. You need some relax, and …. find comfort, for I think, the reward of success. Luckily, still everyone has his own comfort zone, and can we find interesting differences between people. Otherwise we could be programmed! E.g. reading emails or drinking booze or smoking, or …….. Are we not all trained!?