I don't mind sharing food as long as I'm expecting it. It would annoy me if somebody helped themselves without asking to food that was on my plate. I quite like offering drinks and snacks to guests, though I can easily forget that it's a sociable thing to do.
I see a couple of people have mentioned the situation where you offer to get somebody their own portion of some food or other, and they decline it but say they'll have some of yours. Dad (likely an Aspie) once described just such an incident to me, and made it clear that he thought the woman concerned must have been an irrational jerk for showing such a senseless preference. Is it a girl thing? If so, maybe it symbolizes the ancient procedure of the male feeding his mate, or maybe the girl's just watching her weight, doesn't want tempting to pig out, but would like a small taste. It does sound to me like some kind of bonding ritual - if so, it's not surprising that Dad couldn't understand it. He never did get a lot of those funny little social things.
I don't like eating from the same trough like they do in some cultures, though I can see the social value, quite the reverse of the "I don't want your spit in my mouth, I might catch something" vibe. I'd feel OK about sharing a container of food with my partner, using a fork each, because if either of us has an infection then it has other ways of transferring itself between us, but a communal trough for several people seems more risky. On the other hand, I used to share joints with loads of people I didn't always know very well, and never thought about it, but never came to any harm.