Negotiations are challenging in general.
The level of difficulty grows tremendously when you are personally connected to them. If you are the product that's being discussed and across the table is your future employer who has inherent leverage over you it's off the charts.
Why? Your own emotions and fears will cloud your judgement.
Perhaps you've taken coursework in negotiations or you've gone through extensive training. You've learned all about game theory, BATNA, psychology, how to research your opponent and her position. You might even have years of experience in negotiating complex business deals. You feel pretty good about your negotiation skills.
Add a heavy dose of personal emotions and you can throw most of that out the window.
"I'm strong. I know how to control my emotions," we like to tell ourselves. Maybe. But, you're also human. We are all organic creatures ruled by the chemical reactions that take place in response to stimuli.
If you have 3rd party guidance to help you through the negotiation process your chances of success go up dramatically, even when you have training and expertise. With so much at stake, why would you just wing it?