human influences may include genetic, social, and political ones, while non-human influences environmental, natural and physical. needless to say, when you are thrown into burning water, electrified for some secrets, or wounded on the battlefield, it is hard for you to feel happy. also, if you had been born with certain unhappy gene (?), were pressed by your boss to meet the deadline, or were to lose your most beloved one, you might tend to feel unhappy. such stimuli may affect your psychological condition to varying degrees, but they are all external influences, which are out of your control.
internal forces may include your character, knowledge, faith, world outlook, thought pattern and any other intellectual elements that can work powerfully on your psychological tendencies. because these internal influences are acquired and self-developed, they are more or less self-controllable. it is exactly this self-controllability that determines the extent to which you feel happy or unhappy.
as a psychological condition, happiness is in essence a dominant, constant feeling of self-approval, self-content and 100 % self-control?
to achieve a sustainable sense of happiness, one can work only on one's own character, knowledge, faith, world outlook, thought pattern and other significant self-controllable force.