I work around a middle class/high class neighborhood which is very different from where i live @ the corner of freemont st. and the transition from each class your going to see jolly people in the suburbs but omg sometimes you find them really cringeworthy something about their togetherness, family unity and happiness gives me the eliza thornberry's
I think there's a big difference between really being happy and trying too hard to make your life look happy & perfect.
Great movies that examine this phenomena: Revolutionary Road & American Beauty. Both about a person/a couple driven to madness by their mental self-imprisonment of their need to fit in & conform to American middle-class/suburban standards.
Sometimes, it depends on the situation though... If it's just a passing encounter, like you walking down the street and people smile at you, I always find it nice even if it's fake. But if I'm in a bad mood and someone tries to strike up a positive conversation with me, I get really annoyed. It feels patronizing.
When it's not genuine, it's annoying. Somehow you'll know if it's just an act. But really, the negative nancies are so much worse! Better have someone trying to be happy than be with an an emotional black hole.