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!