I will normally share my things, assuming I am able, and that I trust the person borrowing the item. My neighbour borrows my van when they need to transport a group of people, I borrow their car and trailer when I need to take large items to the dump or pick up a load of soil. Of course there are items of great personal value that I may not share or lend, but they are items I doubt anyone would want to borrow.
When my children were little and still learning about sharing the rule was either they were prepared to share the item with their friends, or the item was put away when their friends were over. If we brought toys to the beach or the pool then I expected my kids to share. They were expected to take turns with playground equipment etc. As far as snacks go I would expect my kids to share a snack providing we were in a situation where sharing is appropriate/allowed, and my child had enough to share. When my boys were younger I often brought a cooler full of freezies to the school playground after school so they could share with their friends. I usually do not allow my kids to lend items to their friends, simply because children are generally not good at looking after things and returning things. I have also taught my kids that while I do expect them to share with other that they should not expect others to share with them.
I am very proud of my boys. They are 8 and 11 years old now, and are very good at sharing without having to be reminded. They will offer their friends a piece of gum if they are having a piece of gum, or offer to let a friend have the first turn at something.