As biological living things, we are driven by internal rewards and punishments. The old stick and carrot. Pain is the stick and pleasure is the carrot.
Pain endures. Pain will persist until the source of the pain is removed. Until the splinter is removed, until the diseased limb is cut off, until the slave wins their freedom, until you manage to buy that shiny new car that trumps your neighbour's status symbol, you will feel the stick driving you on.
But happiness is transitory, what Steven Pinker referred to as the hedonic treadmill - You have to run in order to stay where you are. You need new reasons to make you stay happy. You always need more. When you get the cool shoeshine, gratify your desire, what is the evolutionary benefit in sitting there, content, for the rest of the day? None.
After a brief dose of the reward of happiness, we begin to want something else. So we are programmed to look for the next goal. We progress up Maslow's hierarchy of needs. It is our nature never to experience enduring satisfaction. We are machines built to want and want.
Skow makes the case that the neurochemical brain is a dynamic system, and happiness, though a temporary state, is a natural one, so "imbalance" may be too strong a word for it.
Happiness is a temporary chemical imbalance
Yes, those imbalances, those temporary alterations of brain chemistry can make us happy, so very joyous. Nirvana, the abolition of need not by the impossible task of satisfying all possible needs, but by temporarily short-circuiting gratification itself. True inner peace. And it leaves one low and flat afterwards, living for a few days in a world bereft of all the brightness and dimensions that we have experienced. Dull, unhappy.
References: Steven Pinker: How the mind works.