my two theories with downs syndrome people looking happy all the time are:
1) not fully comprehending what is going on, therefore not grasping the dull or unpleasant reality of it all
2) my dad's dog looks sad all the time, but he is asleep and I roll him over - he looks vicious b/c all his floppy skin falls away from his teeth, then if he wakes up and rolls over and looks at me - sad again. a guy I went to college with looked pissed off all the time - not because he was, that was just how he looked.
as humans we are conditioned to see certain facial expressions or movements as denoting signs of emotional response - but it is also possible that they are simply epiphenomena in the scenerio and are a result of how the genes shape the face and/or muscles.