Fundamental Principle

Lastly, this line of reasoning consists, not of debunking current commonsense morality, but of providing a unifying, and underlying theory for them. This argument sees ethical egoism as a fundamental principle of all moral duties.

For example, we have a duty not to lie. Why? Because if we lie, then we would be seen as untrustworthy, and people would avoid us. Oft times we will need people to be honest with us; so if we lie, they feel as if they have no moral obligation to be truthful. It's basically in our best interests to not lie.