Why do my room-mates always have weird bathroom habits?

Well you are in a bad situation right from the get go. I have never liked to share a bathroom because that's when you really become disgusted by the people that you live with. The first thing that I would suggest is that you keep your personal toiletries out of the bathroom. When you need to use the bathroom, then bring your toiletries with you, but remove them when you are leaving the bathroom., this includes your toilet paper. The other thing you should do is buy a bleach cleaner and bring it with you when you go into the bathroom, that way you can spray off the toilet seat and other areas that you need to touch. I do that in my apartment because it is me, my boyfriend, and his father that use the bathroom and I know that my boyfriend's father doesn't clean the bathroom. My boyfriend cleans sometimes. But I always try to at least keep the toilet seat and the sink clean on a regular basis.