Who wrote 'Treat your password like your toothbrush. Don't let anybody else use it and get a new one every 6 months'?

It would seem that you just did...The quote is attributed to Clifford Stoll

The advice makes for a cute saying but fails the analogy. There isn't any reason that you shouldn't let somebody else use your toothbrush. If you know about it and are willing to get a new one it won't cause you any harm. Allowing somebody else to use your passWord, on the other hand, can cause you damage even if that person uses it only once and you change it right afterwards.