Actually...
It's sometimes good to be harsh. It lets the replay maker know that, no, he isn't the best in the world, and yes, he can make improvements. I would absolutely hate it if I didnt receive a single comment asking me to do better, go faster, or get one more dm. Those comments really do drive me to do better, go faster, and GET that last dm. If anything, constructive criticism needs as many negative thoughts as it does positive.