I work with a couple of child psychologists in my group and they really believe in praise and feel that children who are acting out are not receiving enough proper praise.
It is more common when siblings are closer together. Parents will sometimes tend to notice the negative behavior and comment on it and not comment on the positive. It happens when there is a new addition, it is just sometimes too time consuming.
A good balance of praise is to make sure you are complimenting your child more than you are redirecting or correcting. If they see they get more attention and more of a great reaction from those behavior you want from them, they will repeat them. This is very useful in small children. Making a big woop de doo is why they repeat things and want to further explore.
It is like when we take on jobs and responsiblities as adults. The better we do, the more acclimation we receive, the better pay we receive for a job well done. If we take away the praise and rewards, there wont be that little extra insipration to do a better job. For me it would be like wondering what the world be like without the Nobel Peace Prize?