Why do some people do the same things they find gross when other people do it?

For example a person feels using the same towel to clean one’s inner thighs and privates and in-between toes after a shower and then using it to continue drying one’s face is a bit gross and yet he does the exact same thing and is fine about it.

Using the same napkin during dinner to wipe one’s nose and then wiping the mouth with it is disgusting yet the same people who think that way do the same thing themselves.

Can you think of other situations similar to this?