Other things equal, human beings enjoy the exercise of their realized capacities (their innate or trained abilities), and this enjoyment increases the more that capacity is realized, or the greater its complexity.
Rawls' claim is that humans prefer activities that they are already good at and that for any two activities at which a person is equally proficient, she will prefer the one that requires more subtle discriminations. Mill has something like the Aristotelian Principle in mind when he writes:
For Mill, however, we value complexity because it leads to pleasure. But it is pleasure of a different sort than base pleasures—a qualitatively better sort of pleasure. (Hence qualitative hedonism.)
If you feel you've not gotten quite enough exposure to Mill, Rawls and the Aristotelian Principle, you can go read my dissertation.
(I hope you're not actually that bored.)
Hard Times’ Thomas Gradgrind is a caricature of James Mill—John Stuart’s father—the early proponent of utilitarianism, who famously educated his son in an All Facts, All the Time system of education not unlike the Gradgrind school. Like Gradgrind’s daughter Louisa, John Stuart suffered a nervous breakdown in early adulthood, finding his way to recovery through the poetry denied him in his youth. John Stuart's version of utilitarianism would be significantly more nuanced than his father's, incorporating something like the Aristotelian Principle into a conception of qualitative hedonism.