Why do people expect you to share your food?

I don't see anything wrong with not wanting to share. If it's your stuff, then you shouldn't have to feel obligated to let other people use them. I remember one time I was at a restaurant and I had this big hearty meal on my plate. But just when I was about to dig in, my father takes a lump of my fish and puts it on his plate. I was really pissed, and then his girlfriend had to talk to me like I was five about how I needed to learn to share. It's not a matter of sharing, it's a matter of taking things without asking. My dad does that all time. People would ask him if he wants the same thing I'm having. He'll say no, but when I get my meal, he'll always grab some of the food I have. Then complain about how selfish I am. When it comes to food, I shouldn't have to share because it's my stomach and I'm hungry. That would mean less food for me to eat. lol