We are human being. We always try to keep ourselves happy. Happiness supplies us mental satisfaction. Now come to this mysterious word "SATISFACTION".

It's such a word that describes the inner meaning of the entire living system. Our parents mated to get physical satisfaction. Then our birth gave them more mental satisfaction. We were thriving from childhood to teenage by playing, eating, dancing, singing which gave us satisfaction physically & mentally. Then we pass our whole golden, vigorous young age through educating that will bring us money, honors, a pretty wife which will provide you satisfaction. In the long run at the old age we remind of the creator, we worship Him more than childhood, boyhood, young age because of getting heaven in the afterworld and all these things prove that we are the human being as well as all living beings always try to earn happiness, try to quench their thirst that will satisfy their minds....

Scientific Explanation:

When any kinds of living organism take food their body gets energy, blood circulation increases. As a result their nervous system gets more oxygen supply, neurotransmitters exchange increases which is the main scientific reason of mental satisfaction. Not only by taking food but there's have a lot of physical and mental doings that increase the secretion of neurotransmitter such as mating with opposite sex, recreation, having children, superiority, and so on.