i really don't like people being 'familiar' when i don't know them

Sometimes people feel a familiarity because they may be going through the same things, they dont have to know you in order to relate to you. As humans thats what we do we relate to each other through shared experiences, through trauma, even a shared feeling. If you seem sad or bothered it may attract someone else who also feels sad and bothered to you. To answer your question about how to respond, be thankful! That person literally took the time out to acknowledge that you even exist.