It depends on how you would define "sharing".
I'm an in-home daycare provider, and I've noticed that some of the parents have a different idea of sharing than I do. Imagine a parent bringing their child into the daycare, and the other children are all involved with toys/activities. The new arrival wants to play with a toy child X is already using. Suprisingly, many parents will say something like, "I'm sure X will share that with you" to their child, expecting X to hand the toy over. To me that is NOT sharing, and it seems totally unfair.
We do more taking turns. The kids here have gotten pretty good about asking, "May I use that when you are done with it?" We also have duplicates of some of the favorite toys, and large sets of things like blocks, puzzles, etc. and it's understood that everyone can use them together, and no one can have all the pieces to him/herself if others want to play.
Another thing I have taught (or in some cases, am trying to teach) the kids to do is ask if they can join in with a child who is playing with an individual toy/playset. Sometimes they say yes, sometimes no. Either is fine as long as they are kind in their answer.

They are usually pretty good about saying, "I'm playing with this alone."
Sharing can definitely be a tough one, and for some, it never really gets easier.