Why do Caucasians let anyone drink or eat out of their foods?

I notice that most caucasians dont care about sharing drinks or foods that they have already bitten or havent and then eat it after, this has always put me off, I have never shared anything with anyone except for my sisters and parents, but even with friends, I have not done this, I find it unsanitary, I mean, you don't know what they have done, and there is that whole mano" disease. Why do caucasians do this?