Because it's in my interest not to tell them. What do I gain by telling them their idea sucks? I have an uncomfortable interaction and I potentially make an ene

Because it's in my interest not to tell them. What do I gain by telling them their idea sucks? I have an uncomfortable interaction and I potentially make an enemy, or at least someone who now finds it uncomfortable to be around me. What do I lose by being "nice"? Nothing. How does it serve my interests to go out on a limb and tell it like it is?