I don’t believe for a second that John didn’t love Julian no matter how callously he hooked up with Yoko, but for most of his life John ran away when his emotions became too complicated, and his relationship with Julian and Cindy were always too complicated for him to sort out, never more so than in early 1964, on the edge of the biggest thing to ever happen to pop music.
So, maybe John did have every reason on Earth to be sad. He was, after-all, losing the only family he’d ever had…unless, of course, you don’t count The Beatles.