Let’s say I’m hungry and I make a sandwich. Once I eat that sandwich, I’ll feel much better than I did before doing so. My stomach will stop grumbling and I’ll no longer feel quite as hungry. If I then make and eat another sandwich, I won’t feel nearly as good as I did after the first.
If you understood that, you’ve understood one of the most basic principles of economics: diminishing marginal utility. Each additional unit of something provides less utility than does the unit before.