Prosecutors would prefer the potential defendant to plead guilty. That way they can avoid the expense of a trial.
Defendants can be very difficult at this stage because it's extremely stressful. Some don't understand how much trouble they are in.
There is a power imbalance: The prosecutor (the government) has all he power.
Because the government has the power, defendants have constitutional rights.
Because of the power imbalance, the negotiation isn't like in civil cases.
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