I actually think it's a bit rude, TBH. I just eat off my own plate, and if I don't like it then I just won't eat much and maybe just get a snack at home if I'm still hungry.
BUT it is ruder to not accept peoples offers, especially if you're a guest at their house or vice versa, so if they offer it I will take a bite or two. But I wouldn't ask to eat off of someone else's plate.
I think it's just depends on the kind of people you associate with. If they're like my friends, then they would think it's weird if we don't share. It's just the norm. I was kind of raised like that, I guess. I've been sharing meals with my mom for years (huge portions = too much food).
Obviously, some people just didn't grow up that way/ don't feel that way. I don't think there's one wrong or right opinion on this issue.
I steal one of my friend's sunchips everyday at lunch. He's gotten used to it, so he started to buy 2 bags... Now my other friend takes a bag as well. xD
Dunno if that counts as taking food from another person's plate... I don't think I've ever done that without me asking "Are you gonna eat that?" and them saying no.