Why are two-year-old’s mostly playing on their own?

The reasons for this are twofold. First, toddlers can find it hard to understand and to communicate with each other; and second, they are still totally egocentric, self-centred, selfish and notoriously inconsiderate. But they’re learning to socialise, and this process is very much a personality-driven thing too. They will often play in parallel, alongside other children, but primarily solo — otherwise a conflict can ensue.