We yell when we feel like we're not being heard. Maybe the problem is more that our kids hear us too well. Trouble is, our children only hear what we're saying, not everything we're thinking too. So unless we work hard at keeping things in balance, soon all our kids hear are the things that's wrong with them. Soon they feel like there's nothing they do right.
Parents can still find fault no matter what.
As a result, pretty soon the parent feels like nothing they do to help their child be better is working. Just keep yelling. These are often good people. Pretty good parents, pretty good kids. Too many of them feeling worthless and helpless and hopeless to change it for the better.