Somewhere I read that when we are toddlers, we interact in our world in a more genuine manner than we do as we get older. As a toddler, we don't hesitate to laugh, to cry, even to have a tantrum - emotions perfectly appropriate for our current situation.
I also read that at about the age of three, we begin to understand that the important people in our lives expect us to behave in a certain way. We want to please them, so we try to conduct ourselves in a manner that we believe will make them happy.