I have very few pet peeves in my life, but I never, ever share any type of hygiene tool with anyone. Yet, even with a glaringly obvious note that says, "please don't use" on my toothpaste tube, why in all that is green and good would someone use my toothbrush, and even use it right in front of me. I do my best to be a good host, but I would think some things in life are pretty obvious. Especially when there's a note on it.
Protip for others: Never assume you can do whatever you want when you're a guest in someone's home. Just because someone tells you to relax and make yourself at home doesn't mean you have free reign. It means they're doing their best to be polite, even if your dandruff gets all over their couch, your hair sheds all over the floor, and they clean up your dishes. A good guest takes the initiative to clean up so well, it's almost as if you didn't touch anything. This shows respect and honor to your host. Always, always ask before you attempt to use something. It's a sign that you respect their privacy, and are gracious for letting them stay in their private space to begin with. They didn't have to go out of their way, but they chose to.