Why is it?

I have a question, that I'm pretty certain, people will continuously come up with many analogies for, that may or may not make sense, but I'm going to ask anyway.

Why is it that a person, Man or Woman, has to LOSE a person to fully understand how much that person meant to them? As soon as a person gets left with in days, or even hours, suddenly realization strikes how much they took for granted the person who adored and loved them. I don't understand why people cant appreciate and see how much a person loves them by the things they do daily.

Why is it that people don't understand all the good things their spouse brings to the table until they stop bringing it?