Well I don't recall it being said that Dementors feed on happy thoughts but I can see why some one might have thought that. They absorb all happy/good thoughts leaving only sadness and despair in their place and I agree with reapermum as regards to the naming of people. Think on it as B7. Alphas use surnames with acquaintances, first names with close friends and probably surnames with an honorific for seniors or strangers. First names are also used for children and social inferiors for example women(Jenna and Cally)%)