Several skills are necessary to get along successfully with other people. Toddlers, however, have none of these skills. Because of their limited mental abilities, they don't understand that others have a point of view. They certainly don't understand what those feelings or needs might be, and their newly developing language skills don't allow them to communicate well with others. Therefore, they seldom think about how their actions might affect others.
Emotions are also overwhelming to young children.
Toddlers have trouble distinguishing between anger, sadness, and fear. It is almost impossible to control what you don't understand. Furthermore, the toddler has few alternatives to try when attempting to solve problems with others. Therefore, when toddlers are faced with social problems, especially ones that raise strong emotions, their immediate reaction is to respond physically. When they want something, they take it.