"Yes" is what we all strive to make another person say. The objective of negotiating is to inspire or coerce the other person to agree to your terms. Persuading others is the art of the process.
People can be motivated by many things. Simple fear, a desire to be liked, respect for the other person, or simple avarice and greed. Each are motivators in a negotiation.
In most negotiations we assume that the prime motivators are avarice and greed. It is the use of other, less obvious motivators that makes the difference between those who barter and those who negotiate. Bartering is an exchange, typically a fair exchange of like value. Negotiating has the potential of creating value from the process. It is much like making 1+1=11 rather than 2.
There are many persuasion techniques. They all play off the core psychological drivers that effect most people. They represent the basic negotiating tools most of us use consciously or unconsciously.
One such technique is the use of classical conditioning when trying to persuade others. The Pavlovian model can be effective. Ivan Pavlov studied the cause, effect and reaction relationship and how consistent repetition of a reward or punishment can reinforce a specific performance. The important lesson is that the subject need not understand the cause but learns to relate or anticipate the response to the action.