Toddlers can be trying. They're just learning how to navigate the world and don't have a clue about manners or social constructs. They are self-centered because they quite literally don't know any better. They don't mean to be selfish; they just don't know how not to be yet. Sharing doesn't come naturally to toddlers and is very much a learned behavior. There are very important concepts they need to master before they can even begin to understand sharing, which is why I have a very basic rules for talking to my toddler about sharing.
First and foremost, if you aren't me or my husband, you don't get to talk to my toddler about sharing. Unless otherwise stated, if we're around, we'll handle it.
My 18 month old is a typical toddler when it comes to sharing. He declares "MINE!!" with conviction, even while he's grabbing something from his older brother. While it sucks for the 4 year old, I know that my toddler is learning and he's making progress every day, so I don't stress about it. I have very particular opinions on how to teach toddlers about sharing, and one of them is that you don't. Just model appropriate behavior, and they pick it up in time. There aren't many situations where you actually need to intervene, but for when those situations arise, I have tips!
Toddlers are still babies. Be nice to babies. Use empathy. Don't shame. He doesn't understand sharing and isn't capable of understanding sharing yet. We're working on developing those very important, very necessary skills. So in the meantime, ask him if he's ready to give up his toy. If he's not, too bad. We don't take things without permission. This includes adults taking toys from toddlers. You don't like giving up your toys either, so chill.
Ask, Don't Tell
If, for some crazy reason, I have given you my blessing to talk to my kid about sharing, like if you're babysitting, and sharing comes up, ask my kid to share. Don't just take something from him. He's a person. Even though he's small. And you need to show him the respect you'd show any person. And if he says no, that's that. He said no. No means no, even when you're little. You don't just grab things from people. Especially if you're trying to teach them not to grab things from people.
Like, ever. And it's especially not OK for another parent or a total stranger to punish my child in ways that I might not be comfortable with. Punishment and discipline are different things, and I believe that punishment doesn't accomplish much except creating resentment and all sorts of other problems. That being said, when toddlers refuse to share, they aren't being assh*les on purpose, they're just acting like toddlers. Punishing them won't make them be less of a toddler.
Tell Him Other People Will Want To Play With His Toys Before It Becomes An Issue
Toddlers aren't dumb. They understand a great deal of what's said to them, so try explaining that other kids want to play. Again, treat them with respect. Just because they're children doesn't mean they aren't people. Use your words, right?
Let Them Sort It Out Themselves
Many people disagree with this approach, but I say let them figure it out themselves. Don't intervene unless someone is about to get hurt. Don't let it turn into Thunderdome, but give them time to discover how to handle these conflicts on their own. Toddlers are capable of a lot more than we give them credit for and will usually resolve these issues on their own.
Show Them How To Share In Everyday Interactions
And, don't wait till it's go time and you're in the middle of a toddler playdate and expect them to get it. If you don't model sharing in everyday life, they won't have a clue.