Since I shared with you that I do not force my children to share , but instead encourage kindness and generosity  and encourage them to share if they want to , I may as well also tell you that I do not force my children to play with one another.
By this, I mean that I do not believe in forced association.
As adults we are not forced to befriend anyone we find annoying, deceitful, boastful, manipulative, etc. If there is someone we do not care to befriend, we generally have that choice. I understand that there are situations where we tolerate behaviors we do not like, as in if someone works at a nearby desk in your cubicle at the office, but in general adults are not forced to associate with those they do not wish to.
No one makes you go to dinner and a movie with a friend who is a habitual liar, or a chronic complainer, or a manipulative jerk.
This is the same choice I give my children. This usually plays out quite well in our own home. Here is an example of how it might sound:
Child A: "Mommy, Child B won't play with me!"
Mom: "Why not? Were you being annoying?"
Child A: "(hesitating) … no…"
Mom: "Child B, Why won't you play with Child A?"
Child B: "She cries when we don't play what she wants to play."
Mom: "(to Child A) That is annoying. No one likes to play with a crybaby. Go play by yourself or stop being a crybaby and maybe they will decide to let you play with them."
Do you see how this mirrors the real, adult world? If you encounter a new friend who is a liar/braggart/bully/constant complainer/etc, you have the choice whether you want to continue the friendship or not. No one forces you to associate with that person.
Here is another example of how this goes in our home:
Child A: "Mommy, Child B won't stop bossing us around when we are playing house."
Mom: "Then don't play with her. Nobody likes to play with a Bossy Britches."
Honestly, these scenarios rarely happen at our house because my children understand The Law of Natural Consequences. Nobody likes a bully/liar/cry baby/tattle tale/etc. If you act that way, no one will want to play with you.
This is not to say that I allow my children to be rude to others when they exhibit annoying behaviors, and I also expect them to greet new friends enthusiastically, regardless of gender, race, religion, or politics. It simply means that I do not require them to play with children who exhibit undesirable behaviors.
This freedom has worked quite well in our home. I can see in my own children that it has cultivated in them a propensity to be pleasant and get along when playing with others. They know that if they are an unpleasant playmate, they may not be a playmate at all before too long.
They understand the natural consequences of being unpleasant.
I think this is important for my children to learn, not only so they will be pleasant playmates, but also so they will be and look for pleasant life mates.
It won't be too much longer that I expect I will be seeing my own children marry. I hope that I have taught them that they need to choose someone who is pleasant, honest, upright, and amiable as a life-long mate.
They have the choice who they would like to spend their lives with. They have been practicing making good choices of who they would like to associate with all along, so I expect they will choose well.