Exactly. We are forcefully taught to fight these behaviours in ourselves very early on, often before we gain consciousness and memory as children. That will lead parts of our brain to detest and fight those behaviours in others laters on, bot on a subconscious level which makes it hard to change.
Same here. I think this is evidence of genetic factors - i.e. my dad has a mild case and it manifested in him being very emphatic about training us not to make sounds while eating.