From our Texas prosecutor:
I had a discussion with a defense attorney today about the dynamic relationships that develop between prosecutors and defense attorneys. Relationships that, like it or not, become very intricate in the resolution of criminal cases. This conversation started after we had finished watching a heated exchange between a different prosecutor and defense attorney. He started by pointing out how there's not much to gain by creating a hostile relationship with a prosecutor. He described his fear of creating problems for future clients because of bad past relationship with a prosecutor.
I explained how prosecutors are aware of that fear and how our goal is to set aside the personal aspect of negotiations and not to punish a defendant on account of who he happened to hire as his defense attorney. We try and look at the case and defendant separately from the attorney. He agreed he didn’t feel most prosecutors seek to punish the unfortunate client of a defense attorney who recklessly handles business with the prosecution, but quickly added how subconsciously it might be an underlying factor when a prosecutor decides how he's going to handle a case. Again, I stressed we try not to behave that way, but then again, I can't speak for every prosecutor.
Common sense tells you that honey attracts more bees than vinegar and that pissing someone off on a consistent basis might render less than preferable results when it comes to working something out with that person. The personal aspect of dealing in an adversarial system is often too hard for some attorneys to set aside. So, they take things personally. Negotiations are bound to get heated when you deal with one party protecting something (in this case the liberty of their client) and the other trying to take it away. And it should! Criminal cases shouldn't be taken lightly by either side, but passion doesn't have to trump professionalism.
There's not really a how-to on not taking it personal. Just something you have to practice I suppose.