In my opinion and experience, appetizers are intended to share when it is agreed upon before they are ordered. Appetizers belong to the person ordering if they intended to have them for themselves. If you want an appetizer, order one or discuss sharing before someone else orders one. In my family, we usually share appetizers but we discuss it before we order.
Do people think desserts are always for sharing, too? If we all have a chance to order dessert, but only I do, am I obligated to share? If not, what's the difference?