I agree with the gp and the other responders that however much other games might seem like they involve "being a bastard", Diplomacy is entirely another level. Describing exactly how is the challenge.
Let me try: The games' mechanic mean that everyone is just fighting for territory, those who intelligently cooperate will defeat others. So in this situation, the most effective strategy is cooperating intelligently early in the game and betraying your allies mid-game. The insidious thing about the situation is that if you're playing among friends, the natural way to cooperate to leverage the trust you already have with your friends. And so what any winner does is be themselves, use the rapport they have with their to get cooperation but be just a little less trustworthy than their friends expect. That is where the tendency to break friendships appears.
Let me try: The games' mechanic mean that everyone is just fighting for territory, those who intelligently cooperate will defeat others. So in this situation, the most effective strategy is cooperating intelligently early in the game and betraying your allies mid-game. The insidious thing about the situation is that if you're playing among friends, the natural way to cooperate to leverage the trust you already have with your friends. And so what any winner does is be themselves, use the rapport they have with their to get cooperation but be just a little less trustworthy than their friends expect. That is where the tendency to break friendships appears.