While there's no single "right" reason to leave a guild, there is definitely a "right" way to leave: honestly.

I don't expect the people who choose to leave my guild to post an epistle on our forums about why they're leaving, but I would rather be told that they're quitting because of reason x (want faster progression, can't raid the schedule anymore, has issues with player y, etc) because then I have received valuable feedback to use when recruiting (am I somehow misrepresenting the guild, is a current guildy a potential problem, etc).