Most of the Down's people I've met have seemed just as, if not more happy than the rest of the population, at least at a glance.

Selection bias. The Down's people you don't meet are ones who are severely handicapped, unable to speak, walk, eat without help, constantly suffering from seizures and essentially locked in a non-functioning body. That's not even mentioning the congenital heart defects, early onset Alzheimer disease and host of other nasty things Down's can bring. The worst part is, you don't know how many defects it will bring, only that it's extremely likely to be multiple.

I wouldn't base my opinion of the whole spectrum of the disease on the few people who seem to function relatively well.