It is common to wish someone, especially children, "pleasant dreams" when they go off to bed.  That seems like a good thing to do.  We don't want kids to have scary nightmares.  But is that always the best policy?  Maybe not.  When we have pleasant dreams, we generally pay little attention to them unless they are very unusual in some way or we have the same dream many times over.  On the other hand, if we have a nightmare, we are more likely to wonder about it, to try to understand it, to figure out it's meaning.
    This is really a very normal thing to do.  If no one says anything about the way we dress, for example, we will probably continue to dress the same way.  If someone makes fun of the way we dress, we will likely try to change.  Especially kids who always want to fit in and be popular.
    We should pay attention to our pleasant dreams as well as the nightmares.  All dreams can provide information to us.  Some may be about things in our present life that we are not aware of.  Some are about the past and some are about the future.  As a general rule, dreams about the past occur shortly after you fall asleep, dreams about the present occur in the middle of the night, and dreams about the future happen in the last hour or two of sleep.  Those scary nightmares are often about the future and, when you learn the symbolic meaning of what happened in the dream, you often find that they are not as scary as you think they are.
    Not all dreams are significant, of course.  Some have no real meaning at all.  They may just be the result of something you ate, for example, eating a bean dish in the evening can lead to scary, but meaningless nightmares.  Many dreams are significant though, so you should try to interpret them by getting yourself a good dream dictionary.  There is also a good discussion group that lets members analyze each others dreams.