Philip Brickman, a social psychologist at the Northwestern University in the U.S., had an experiment. It was to observe changes in emotions of those whose happiness greatly increased as they suddenly became rich through an unexpected fortune and of those who became unhappy drastically because of paralysis on their body owing to a sudden accident.
When the degree of their happiness was reexamined after some time, the result was contrary to the first state. As time passed by, the high level of happiness lowered to the state of before they had met the fortune, and the low level of happiness was restored to the state of before they had the accident.
Human brain has a habit of returning to its familiar previous state of emotion as time goes by even if it goes through emotional changes by certain stimulation. Because of this, the brain that is accustomed to negative emotions puts more meaning on what caused negative emotion among what occurred a day, and remembers it longer. On the contrary, the brain that is accustomed to happiness receives pleasant things more than unpleasant things, and when it looks back a day, it recalls happy things first.
Repeating is the only way to obtain the habit of having good emotions such as joy and thanks, throwing away negative emotional habits. That is why God asks us for this:
“Be joyful always; pray continually; give thanks in all circumstances, for this is God’s will for you in Christ Jesus” 1 Th 5:16–18