This is one of the key insights in playing Diplomacy well. You can turn honest opposition in the early game into trust, which is helpful for flipping allegiances in the late game. ('Yes, I've been trying to murder you the entire game, but I've been honest about it, and never lied to you once. Now I see that we've got a real opportunity to dislodge Russia and sweep the eastern half of the board if we work together... What do you say?') Dishonesty means people don't want to work with you, because you are not a dependable partner. So you gotta make sure that any lies you tell pay off /big./