Diminishing marginal utility means that as you keep consuming the same thing, each extra unit gives you less satisfaction. For example, if I eat my favorite donut, the first bite is delicious. The second is still good. But by the third or fourth donut, I feel full or bored. This happens in shopping too—buying your first new outfit feels exciting, but the tenth one might not feel special. This concept explains why people stop buying after a while and why producers offer variety or promos to keep people interested.