When I'm eating with friends I consider it perfectly normal to share food, and I enjoy being able to try different things. I especially like eating out family style, so you get lots of different choices, and if you have everyone order a dish of their choice. They get their favourite + variety.

As far as offering food to strangers/receiving food. I'm usually a little less willing to do, but frankly its not that big of a deal. I'll offer my food on an airplane if its sealed--like a yogurt, or a sealed snack etc. But I won't offer something uneaten off of my plate.