BECAUSE IT'S THEIR BODY NOT YOURS.
YOU ARE ENTITLED TO NOTHING.
Ok, so maybe I'll explain a little more.
Consent is generally thought about in terms of sex. However, consent is still a necessary concept for any action that can invade a person's space, body, and sense of well being. And since most people do not know what actions will invade someone else's sense of well being, it's best just to get consent before any action AND NOT GET OFFENDED IF CONSENT IS NOT GIVEN.
Oh, that person didn't want to shake hands with you? Brush it off. For all you know, they are sick or are insecure about their really cold hands.
That girl didn't want to hug you? She probably doesn't feel comfortable with you and/or doesn't feel safe.
Do not blame or shame people who do not want to be touched. You have no idea why they don't want physical contact. That person could be a survivor and physical touch is a trigger. They could have a sensory disorder where touch is to over stimulating. Or they could simply not want to be touched. It is their body and they have the right to decide who has access to it or not.
