Adults ask toddlers to share long before sharing is available to them. The request assumes an understanding of possession and return that develops considerably later than the language for it.
Ownership is understood before generosity
Some time in the second year children begin asserting that things are theirs. This looks like selfishness and is a cognitive advance, since it requires a stable idea of possession.
Sharing depends on that same concept plus an additional one: that an object given away can come back. Without the second, sharing is indistinguishable from loss.
A toddler resisting a request to share is usually not being unkind. They are protecting something they have no reason to believe they will see again.
Turn-taking supplies the missing piece
Turn-taking makes return explicit and time-limited. The object goes, something marks the interval, and it comes back, which demonstrates the principle rather than asserting it.
An external marker helps because small children cannot judge duration. A timer, a song, or a count that both children can hear removes the adult from the role of arbiter.
After enough repetitions the return becomes predictable, and children begin offering turns without prompting. That is the point at which sharing starts to be real rather than compelled.
Enforced handovers teach the wrong lesson
Taking an object from a child and giving it to another on request teaches that possession depends on adult attention and that complaining loudly is effective.
It also interrupts the first child mid-activity, which is the actual source of the distress. The interruption, not the object, is usually what produces the reaction.
Letting a child finish with something before it passes on is slower and produces less conflict overall, because the waiting child learns that turns arrive reliably.
Play is parallel before it is joint
Toddlers commonly play alongside each other with similar materials rather than together. This is a normal stage and not a sign of poor social skills.
Conflict over objects is high in this period precisely because the children are near each other without a shared plan. Duplicates of popular items reduce it considerably.
Genuinely cooperative play, with agreed roles and a shared goal, develops later and brings its own negotiation. Sharing becomes easier once there is a joint activity to protect.
What actually accelerates it
Children share more readily with familiar peers, when they are not tired or hungry, and when they have been asked rather than instructed.
Naming what the other child wants, rather than issuing a demand, gives the toddler information they cannot yet infer, and is more effective than an appeal to fairness.
Adults sharing visibly and describing it matters more than instruction, since generosity is largely acquired by observation at this age.