Rapport is not essential to a negotiation, you are not there to win friends.
However, think about it: the deal is much more likely to be successful if you are friends. Friends are much more likely to help you achieve your win, friends are much less likely to deceive you or get their win at your expense.
Robert Cialdini, Professor Emeritus at Arizona State University, has done extensive research into the world of influence and documented his findings in the book, Influence: The Psychology of Persuasion. He found six key rules of influencing and one of them is getting the other person to like you.
I am not saying become best friends with your counterparty. But I am saying that you will get a better deal if they like you. One ...