As a negotiator, your goal is to have more influence on the other side than they have on you. Your goals and objectives must be seen to gain merit while the goals and objectives of the other side lose merit. This means your presentation of "facts" and other evidence must be convincing. You must also create some doubt in the mind of the other negotiator about the validity of his/her position. In essence, you will be influencing that person's belief system, about both your position and his/hers. A positive relationship with the other negotiator is essential if you are to have this kind of influence. This sounds difficult, especially in view of the fact that negotiation has the potential for conflict. However, experienced negotiators emphasize that it is both possible and required.
A positive relationship makes possible the development of common ground; in principled negotiation the common ground can include similar goals and objectives. Instead of negotiating against each other, the negotiators form a team and negotiate "against" the problem. They now have shared interests.