Why do I get treated differently by how I look?

I’ve been noticing this for awhile now when I’m out places I get stared at or people would always tell me I look sad/mad but I can’t help it, that’s how my resting face is. But I hate it because it seems like it draws people away from me where they wanna avoid me at all times. It has happened at school, work, grocery stores etc. And when I’m out with family people speak to them normally with enthusiasm, but when they turn to me all I get is a dry hello or they say nothing at all. And I don’t wanna walk around with a grin on my face just to look approachable because I don’t want to look weird or creepy and I feel like people shouldn’t judge by looks. I could be happy on the inside but my facial expression doesn’t show it sometimes. But stuff like this ruins my confidence and I have social anxiety too which has worsened because I’m afraid of how I appear to people in a negative way.
Why do I get treated differently by how I look?
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