It is important to remind children that negativity does not have a higher value than positivity, especially in today's world where answering "no" is often easier than answering "yes."
We all know we can't really know what our kids are like when we aren't around, so if you see a kid being a wonderful human being, make sure to let their parents know just how wonderful they are. God knows there are enough people around to let them know when their kids are being lousy.