Your brain is full of chemicals, among them, a neurotransmitter called dopamine.
How much dopamine you have saturating your gray matter can affect how you think and feel. For example, having too little dopamine can cause disjointed, irrational thinking and memory loss. Dopamine shortage is commonly linked to schizophrenia.
Scientists have discovered that dopamine also has something to do with human happiness. Specifically, how much, or how little, dopamine you have to a large degree determines how happy it makes you feel to achieve goals and win rewards.
At some basic level, humans are motivated by things like sex, food, money, and other rewards for work and achievement. Not all people derive the same amount of pleasure from incentives and rewards, though. A person whose brain produces a lot of dopamine, and whose receptors are highly sensitive to the chemical, is bound to be more motivated by incentives and derive more pleasure from rewards. A person with lower dopamine levels is less likely to experience or expect happiness upon grabbing the brass ring or winning the girl or guy.
There are always exceptions, of course, and happiness comes in many forms and has many root causes. Other brain chemicals, most notably serotonin, also play important roles in how we experience happiness and other emotions. Plus, winning a promotion at work might fill you with glee until the long hours and added responsibilities of your new position start making you miserable. No matter how much dopamine you have, happiness may still be fleeting.