I'll admit I didn't know that one either. I found this on dictionary.com.


Usage Note: The verb enthuse is not well accepted. Its use in the sentence The majority leader enthused over his party's gains was rejected by 76 percent of the Usage Panel in the late 1960s, and its status remains unfavorable: the same sentence was rejected by 65 percent of the Usage Panel in 1997. This lack of enthusiasm for enthuse is often attributed to its status as a back-formation; such words often meet with disapproval on their first appearance and only gradually become accepted over time. But other back-formations such as diagnose (a back-formation from diagnosis that was first recorded in 1861) and donate (first cited in 1785 as a back-formation from donation) are considered unimpeachable English words. Since enthuse dates from 1827, something more significant may be overriding the erosion of popular resistance. Unlike enthusiasm, which denotes an internal emotional state, enthuse denotes either the external expression of emotion, as in She enthused over attending the awards ceremony, or the inducement of enthusiasm by an external source, as in He was so enthused about the diet pills that he agreed to provide a testimonial. Possibly, some people's distaste for this emphasis on external emotional display and manipulation is the source of unease that is manifested by a distaste for the word itself.