No doubt it is helpful to have good personal relationships with foreign leaders. However countries have to deal with each other even when the chiefs of state dislike each other.

I think there has been a historic tendency for US Presidents to think that the personal characteristics that made them effective in domestic politics (things like charm, charisma, ability to persuade - the sort of characteristics most Presidents have at least to some extent) could be carried over into international negotiations. I think they have sometimes found stern faced foreign diplomats and leaders a bit of a challenge.

Tony Blair is also an example of someone who has found his personal characteristics were not enough to be a successful diplomat. His failure to make President Chirac move on farm subsidies (totally predictable to anyone who understood Chirac's political history and domestic priorities) demonstrates the limits of Blairplomacy.