[C]ooperation in chimpanzees is highly constrained. Chimpanzees will cooperate only with familiar group members, with whom they normally share food. If they don't know or like a potential partner, they won't cooperate no matter how much food is at stake. Humans, however, make a living collaborating, even when it's with people they don't know and in many cases don't particularly like. (Do you have a boss?) This high level of social tolerance is likely one of the building blocks of the unique forms of cooperation seen in humans. So perhaps a lack of tolerance is one of the main constraints on chimpanzees' developing more flexible cooperative skills.
by Patrick Appel
Vanessa Woods and Brian Hare have a fascinating article on our closest relatives. A taste: