I just don't understand _what it does_that prevents so many Americans from being as happy as one might expect.
what it does 此处是宾语从句,作动词understand的宾语,意为：什么事情使得.；that prevents so many Americans from being as happy as one might expect 是定语从句,修饰what.
此句的意思：
我只是弄不懂到底是什么让如此多的美国人不象人们想象的那样高兴.