Infants and toddlers are exploring the world and learning how things work.  A child who grabs a toy might not know how that toy works until he sees another child using it.  A toy or object may not seem interesting until he sees how it can be used. 

Infants and toddlers want what they want when they want it.  This means that they have a hard time with following rules, sharing and taking turns in particular.  A child who grabs a toy has little self-control, which means he is not very good at waiting and struggles to stop himself from acting on his desires.  He needs lots of practice and demonstration to master the social skills of sharing, taking turns, and resolving conflict before he will learn the joy of friendship.