Being told to be “nice” is like being told to be “good.” It doesn’t mean much unless it is explained.
Details are needed so children know what they did that was nice or good. Then they know what to do again in the future. If they don’t know exactly what you expect, they are left trying to read your mind. Reading minds is not a talent most children possess.
Children do not easily know what nice or good behavior is in any given situation. Does being nice mean giving in all the time? Can you stick up for yourself and still be nice? Can a person be nice and still get her needs met? Is “nice” just for girls? Is there a nice way to say “no?” Can nice people insist on things? Can they be loud? These are all important questions that over time children need to collect the answers to as they grow up – with the help of the adults around them.