Watching your child play might look simple. It isn't. Kindergarten and early elementary is where kids are first learning the difference between playing near someone and playing with someone, and that's a bigger jump than it looks.
From parallel play to real play
Younger kids often play side by side without much interaction: two kids, two separate worlds, same sandbox. Around this age, that starts shifting toward real back-and-forth play, sharing an idea, taking turns, building something together on purpose. That shift doesn't happen all at once, and it doesn't happen at the same pace for every kid.
Why sharing is hard, not just a discipline problem
"Use your words" and "you have to share" are common refrains at this age for a reason. Turn-taking and sharing require a skill your child is still building: holding their own want in mind while also tracking someone else's. That's a real cognitive task, not a character flaw. Expect it to be inconsistent for a while yet.
The "you're not my friend anymore" moment
Friendship at this age can flip in a sentence. Best friends at 9am, "not my friend anymore" by 9:15, best friends again by lunch. Kids this young mean something different by "friend" than adults do: less a fixed, permanent bond and more a word for whoever they're playing with right now. Not cruelty, not instability, just a word that hasn't settled into its adult meaning yet. It stings to hear, but it usually isn't as serious as it sounds.
What helps
Narrate what you see rather than fixing it for them. "Looks like you both wanted the red truck. That's a hard one." Naming the actual problem, out loud, in simple terms, gives them language they'll eventually use themselves. It also happens to be a specific, teachable skill, not just a parenting instinct you either have or don't.