I've traveled solo to France, Spain, and the Netherlands, and to many cities in Italy and have never encountered a problem getting a table at a restaurant.
I am, however, getting the disquieting feeling that some (many?) restaurants in Naples simply do not welcome solo diners. One restaurant, for example. doesn't allow you on it website to check off "one" for the number of patrons seeking a table. It only allows the number "two" or, of course, more. I've even read reviews of restaurants in Naples that suggested that even couples were not exactly embraced because the restaurant was interested in quick turnovers. ("Next!")
Am I wrong about this? Has anyone had problems in Naples as a solo diner? As part of a couple? Could it be that the problem, if it exists, is peculiar to pizzerias as opposed to restaurants that offer a more diverse menu?