Harmony of Interests:When the interests of individuals are aligned such that a benefit to one is generally a benefit to the others.
A commonly held belief is that people's interests do conflict, and that one man benefits only by harming others. This could be based on a zero-sum view of the world, where there's a finite amount of wealth to go around. But whatever the reason, the view is that there is some fundamental conflict of interests that requires us to either sacrifice others to ourselves, or sacrifice ourselves for others. This view is mistaken, as we can and do benefit from other people.
Similarly, people see opportunities where they can acquire some benefit at the expense of another, for instance stealing some money from him. They then suggest that this means there's a conflict of interests, as both parties want the money, and only one can have it.
The Harmony of Interests is a view that people's interests do not conflict, and that in fact we benefit from other people. We benefit from having a social system that allows peace and cooperation. While we may immediately benefit from stealing from someone, the costs would be significantly higher. If we pursued such an action, we lose all of the benefits that come with society, such as education, increased wealth, friends, lovers, safety, and more. The costs would vastly outweigh any momentary gain.
When you understand what your rational interests are, you can see how they are in harmony with the rational interests of others.