diminishing marginal utility
The first bite of cake on a birthday is heavenly. The second is great. By the fifth, you are slowing down, and you would not dream of a tenth. This everyday pattern, where each extra unit of the same thing pleases you a little less than the one before, is so common that economists call it the law of diminishing marginal utility.
Stated carefully, the law says that as you consume more of a good in a given period, holding tastes and everything else fixed, the marginal utility of each additional unit tends to fall. If three glasses of lemonade give marginal utilities of 9, then 5, then 2 units, the falling numbers are diminishing marginal utility in action. It is about the rate of extra satisfaction, not the total: your overall enjoyment may still be rising even as each new glass thrills you less. The effect is usually strongest over short periods, when you cannot fully adjust the rest of your spending or your hunger.
This single law does a remarkable amount of work. It explains why demand curves slope downward, since you will pay less for a unit that gives you less added satisfaction, so a seller must drop the price to coax you into buying more. It also underlies the idea behind progressive taxation arguments, that an extra dollar may matter less to a rich person than to a poor one, though that comparison across people is contested precisely because utility cannot be measured or compared with certainty.
A free upgrade to a second monitor delights you; a third monitor is mildly useful; a fourth just clutters your desk. Each extra screen adds less satisfaction than the last, the classic shape of diminishing marginal utility.
Each added unit of the same good tends to satisfy a little less.
It is a tendency, not an iron law. Collectors, addicts, or a person assembling a matched set can feel rising marginal utility for a while; diminishing returns is the usual case, not a universal one.