It is not good manners to tell people that their idea, in your opinion, "sucks". That's why. I wouldn't tell them that, just as I wouldn't tell them that, in m

It is not good manners to tell people that their idea, in your opinion, "sucks". That's why. I wouldn't tell them that, just as I wouldn't tell them that, in my opinion, their clothing, haircut, or other aspects of personal appearance "suck". Nor that any aspect of their personality "sucks".