An infant or young toddler who hits is not yet aware that it might hurt—it may even be playful. He does not have an awareness of his own body or an understanding that he is a separate person—so he is not yet able to empathize or recognize that hitting another person might be harmful.
An infant or young toddler doesn’t know his own strength. He does not automatically understand the differences between a kiss and a bite, between a pat and a hit, between a nudge and a push. He needs many reminders and he needs to experience gentleness physically in order to learn how to be gentle. “This is how you can touch the baby, softly;” or “It hurt when you hit me, I like how you are patting my arm instead.”