Didn't know such a person can exist... I would say the person is fake and putting up a front and cries him/herself to sleep often? Just pretends to constantly be happy to hide the real sadness...
Sure they can, but it is a very rare personality type. The lower edges..........are the same as anyone else's, but upbeat to an extent, so much more of the time, you really wonder if there not just doing eccies (exstacy)all day. What gets really disturbing, is when you find out there not, and you can no longer explain away this subversive, happy behaviour. They do exist, absolutelely.
On Grey's anatomy there was a newlywed woman who was constantly happy. Her serotonin level was high all the time due to a tumor... That would be the opposite of depression. Euphoria as it was mentioned...
"While a person's overall happiness is not objectively measurable, this does not mean it does not have a real physiological component. The neurotransmitter dopamine, perhaps especially in the mesolimbic pathway projecting from the midbrain to structures such as the nucleus accumbens, is involved in desire and seems often related to pleasure. Pleasure can be induced artificially with drugs, perhaps most directly with opiates such as morphine, with activity on mu-opioid receptors. There are neural opioid systems that make and release the brain's own opioids, active at these receptors. Mu-opioid neural systems are complexly interrelated with the mesolimbic dopamine system. New science, using genetically altered mice, including ones deficient in dopamine or in mu-opioid receptors, is beginning to tease apart the functions of dopamine and mu-opioid systems, which some scientists (e.g., Kent C. Berridge) think are more directly related to happiness. Stefan Klein in his book "The Science of Happiness" links these biological foundations of happiness to the concepts and findings of Positive Psychology and Social Psychology."