I think part of it is the need to come out on top - not necessarily a pride/ego thing, but just wanting to win the deal. Look at pro sports - a lot of trades don't blow your mind, but we all want to pull of those rare trades that work hugely in our favour. If you make a trade that's totally even - it's almost like a tie. "Kissing your sister" so to speak. Sure, you may get the players you want, but at the end of the day, you don't feel like you "won" and so you second guess.
As well, we all tend to start our trade talks at an extreme and work towards the middle. If a guy agrees too easily, you start to think "wait a minute - is he fleecing me here? That wasn't as hard as it should have been. I better add more to the deal."