I think I get what OP is getting at. There's a difference between sharing your chocs around because you WANT to, and sharing your chocs around because you feel like you HAVE to. OP never said anything about not wanting to share ever - they said they don't want to be MADE to share, they don't want to feel like they're EXPECTED to share.
Possession is nine tenths of the law, after all. Once someone gives you something, that gift then belongs to you, whether it be a gift card, a box of chocolates, or a rocket ship capable of interplanetary flight. The giver does not have the right to dictate what the recipient then does with said gift. If the recipient WANTS to share out chocs or offer rides to Mars, that's the recipient's choice. All this pressure on people to share, share, share... the shoe would be on the other foot if people were helping themselves to your things without asking, I guarantee.
It is especially unfair on children, as a PP said. If I share my chocolates and they get eaten, I can nip down to the shops and buy some more. A child doesn't have that ability, so for someone - whether it be another child or an adult - to expect that that child should give up part of their gift is pretty cruel. I've got this mental picture of an adult giving a child a box of chocolates and then sternly telling the child "it's good to share things" and scoffing the lot, which I realize is an exaggeration of the topic at hand but it's still getting me riled up.
We teach our children to share toys and such because toys and such aren't consumable objects. They will, presumably, be there later after the other child has had a turn. Food is different, I think.
I don't think anyone, child or adult, should be MADE to share a gift that has been given to them. Encouraged, perhaps, but never forced or made to feel obligated. If you model sharing behaviors, those are the behaviors you are likely to see back. If you start forcing it, you'll get resistance and resentment.
And for the record, I don't see the OP or their attitude as selfish. Everyone has things that belong to them, and everyone has a right to expect those things to continue to belong to them. To all of the people on this thread calling OP selfish because they don't think they should be EXPECTED to share a gift given specifically to them, well, I'm going to go ahead and assume none of you lock your doors and windows. Because it's good to share, right? So what right have you got to keep people from sharing your belongings?