Fortuitous has developed in sense from “happening by chance” to “happening by lucky chance” to simply “lucky, fortunate.” This development was probably influenced by the similarity of fortuitous to fortunate and perhaps to felicitous: A fortuitous late-night snowfall made for a day of great skiing.
Many object to the use of fortuitous to mean simply “fortunate” and insist that it should be limited to its original sense of “accidental.” In modern standard use, however, fortuitous almost always carries the senses both of accident or chance and luck or fortune. It is infrequently used in its sense of “accidental” without the suggestion of good luck, and even less frequently in the sense “lucky” without at least a suggestion of accident or chance: A fortuitous encounter with a former schoolmate led to a new and successful career for the artist.