In the first sentence, "but" is used like "other than". "Why should a sequence of words be anything but a pleasure?" meaning that the speaker sees no reason for a sequence of words to be anything other than pleasure. A sequence of words is pleasure and nothing else. Both of those sentences are a little awkward, however: "I love nobody but you" means "You are the only person I love." "I love anybody but you" means "You are the only person I don't love." Again, "but" is being used in the "other than" sense. More natural ways to say those sentences: "I don't love anybody/anyone but you" or simply "You're the only one I love." "I love everybody/everyone except you."