This theory can be tested at Culturomics' sister site, the Book Ngram Viewer. Simply type in a word or phrase to see how often it was used in a certain time period. Sadly, the use of both "love" and "joy" has sharply declined since the early 1800's. But not to worry..."in a pickle" is holding steady, and "for crissake" (which first appeared in the 1930's) is experiencing a tremendous rise in popularity.
But I think where it gets more interesting is when you start to compare the same word in different languages. For instance, in French and German literature, the word "love" is on the rise. In Russian lit, "love" had a crazy peak in the 1920's, but hasn't been used much before or since. And in Chinese, it wasn't even seen before the 1920's. Strangest of all, Spanish language books used the word like crazy in the early 1800's, but not much since.
So are we less loving than the Europeans? Did something happen in the Twenties to spike Russian love or introduce it to Chinese lit? And why did love disappear from Spain? While the data is interesting, conclusions are hard to find.