We're guessing you imagined some flighty character from a sitcom or romantic comedy. Someone who's always smiling and waving at strangers, dancing in the rain because they like the way it feels -- you know, taking pleasure in the little things, not worrying too much about the world. They're endlessly optimistic, and always sure that everything will be okay. They're probably an artist or some s**t.
They're definitely way too into balloons for an adult.
And we're guessing that you are imagining this person as being A) very childlike and B) kind of stupid. In other words, you secretly don't think they're really happy -- they're merely oblivious and/or suffering from some kind of brain disease. They're happy in the way that a baby or a dog is happy: because they don't know any better. After all, it's the brooding geniuses who drink themselves to death or commit suicide. Not only because they can see the tragedy of the world ("I'll have you know that slow loris at the beginning was probably kidnapped and tortured by poachers!"), but also because they can see all the ways they're screwed up -- self-awareness means full knowledge of your own failings.