For a negotiation to be 'win-win', both parties should feel positive about the negotiation once it's over. This helps people keep good working relationships afterwards. This governs the style of the negotiation – histrionics and displays of emotion are clearly inappropriate because they undermine the rational basis of the negotiation and because they bring a manipulative aspect to them.
Despite this, emotion can be an important subject of discussion because people's emotional needs must fairly be met. If emotion is not discussed where it needs to be, then the agreement reached can be unsatisfactory and temporary. Be as detached as possible when discussing your own emotions – perhaps discuss them as if they belong to someone else.