Learnt the concept of marginal utility from Economics. Marginal utility is the benefit you accrue by doing/having/seeing/eating that extra thing. As you keep doing/having/seeing/eating it more, the benefit derived from doing/ having/ seeing/eating it goes down. Marginal utility always diminishes and will get into negative territory.

Example, at a stretch if someone who likes colas is asked to drink cola, the following will be the reaction: First cola is good. Second, is okay. Third, nope. Fourth, hate colas.

Marginal utility has good practical application. It for instance can explain the story of marriage.

When you fall in love with someone, your heart thumps, you want be with the person. All the time, yes. You want to know everything about the person. Everything that person does impresses you.You are willing to do anything for that person.

You manage to marry that person. The Marginal utility comes down. Heartbeat is quite normal. You are okay with spending at best the evenings or/and nights with that person. You start finding faults, which were earlier ignored as inadvertence.

Two years into marriage. Marginal utility is still down. Heartbeat? Why? Have time to spend evening only once in a week. You watch TV or pursue your hobby for which you never had time. You find faults, even when the other person was right. It was the person’s nature, but what do you care.

Ten years into marriage. Marginal utility is pretty low. I need space. I want to do my own thing. No time for you. I am really bored seeing your face. Perhaps, let us go out on vacation. Separately, that is.

The more you see of the person and the more you interact with the person, the less the fun the person can give. Diminishing marginal utility.

Post Scriptum:
=The marriage defined here are not necessarily applicable to all or/and all times.

= Marriage can be explained using other theories as well. For instance, (a) familiarity breeds contempt; (b)availability of a person makes people take that person for granted (c)satisfied want no longer motivates.