There's an old jewish saying "When you look in the mirror, you see your best friend" and I have found this to be very true. I am my best friend. Nobody knows me as well as I know myself, my history, feelings, wishes, my strengths and weaknesses... but what do others see?
In the past, I have found it to be like this:
People try to see themselves in you.
They try to see characteristics that they can put into little boxes to know how to handle you.
They try to see other people they "knew" in you, to feel more comfortable around you. That way, they think they don't have to start fresh, they can start with pre-formed opinions.
They apply stereotypes to you, to be able to brush you off as "one of those".
(Almost) noone really tries to look at you as a new person.
Someone who hasn't been on this world before.
Someone who doesn't necessarily have to make the same mistakes you made or the people before made.
Someone who brings something new to the table, where you can't say "Oh, you're like this guy I knew. Oh, you can do this thing I've seen before. Oh, you remind me of myself, when I was young".
Just someone. Not anyone.
It is human nature to try and classify people we meet, because we want to make it easy for ourselves. It's one of the biggest mistakes we can possibly make and one of the most offensive ones, because it robs us of the possibility to connect with someone on a level where someone is, rather than seems to be.