Antipathic personal relations among world leaders
As in any interpersonal situation, presidents and ministers of countries can find their relations to their counterparts in other countries totally undermined by personality issues. This can have a very direct impact on their ability to come to agreement on issues that confront them.
With very rare exceptions, politicians are seldom alone with each other. Their meetings are planned and rehearsed by top officials by whom they are constantly surrounded. Diplomacy is designed to obviate the unpredictable consequences of spontaneous interaction.