Self-discipline, which walks hand-in-hand with perseverance, is a key ingredient in the formula leading to success and happiness. In his book “A Guide to the Good Life: The Ancient Art of Stoic Joy,” William B. Irvine again and again examines the advantages bestowed by self-control. His chapter titled “Self-Denial” addresses the benefits the ancient Stoics found in denying themselves certain pleasures—for example, abstaining from eating rich foods in favor of more common fare and finding rewards in that act of denial. “Water, barley-meal, and crusts of barley-bread,” Seneca tells us, “are not a cheerful diet, yet it is the highest kind of pleasure to be able to derive pleasure from this sort of food.” Unfortunately, ours is more an age of self-gratification than of self-denial. We see running shoes in a store at the mall, decide we absolutely must have them though three other pairs are collecting dust in our closet, …