What are three ways in which humans depend on interaction with one another?

A popular saying is that "No man is an island." This is true on many levels. People depend on interaction with each other in terms of commerce and trade. It is hard for one person to be able to satisfy their own wants and needs on their own. People need to be able to either trade for what they want or to be able to spend money to obtain the goods and services that would make them the most comfortable.
People also need some sort of governance; this usually entails having both rights and responsibilities to the government and one's fellow citizens. Without law and order, power would go to whoever was best armed or strongest, and people would live like animals.
Finally, people need warmth and socialization in order to be healthy. Studies have shown that older people live longer and better lives when they are able to maintain their social circles—be it through family, work, or hobbies. Children learn language and social norms by interacting with each other. Without these interactions, people would not know how to interact since interacting in polite society is not innate behavior, but learned behavior. People need social interaction for cognitive development as well as to develop interpersonal skills.