Diplomacy has existed since the beginning of the human race. The act of conducting negotiations between two persons, or two nations at a large scope is essential to the upkeep of international affairs.
According to my opinions, the first thing is " transparency". If doesn't have transparency on the process of diplomat, the relationship between each country might be difficult to believe vice versa.
The second is " Security". If that state or country does not have internal and external security, the diplomacy is difficult to build between each other.
The other points are : Freedom , Welfare and Justice.
Let me know your thoughts. Welcome any comment!