Intensity.

Happiness is often presented as being very dull but, he thought, lying awake, that is because dull people are sometimes very happy and intelligent people can go around making themselves and everyone else miserable. He had never found happiness dull. It always seemed more exciting than any other thing and capable of as great intensity as sorrow to those people who were capable of having it. This may not be true but he had believed it to be true for a long time and this summer they had experienced happiness for a month now and, already, in the nights, he was lonely for it before it had ever gone away.

Ernest Hemingway, from Islands in the Stream