People are happy when..
-they want to be happy
-they don't know any better (ie they don't know what they're missing)
-the do know what they're missing, but they have a naturally sunny disposition and don't care.
-they're high on drugs

to quote David Hume,
"He is happy whom circumstances suit his temper; but he is more excellent who suits his temper to any circumstance."

and then there are some factors that can influence people's view on life, like the amount of seratonin in their brain, and the like.