So they say, because there is such a belief that if you tell a bad dream before 12 noon, it will come true. Although, on the contrary, from childhood I heard that before 12 it is necessary to tell a dream so that it does NOT come true :)
I think this is all in order to calm children: "tell me what you dreamed and the nightmare will definitely not come true ".
In general, if you had a nightmare, look out the window and say" the night is over - the dream is gone "+ smile. Also one of the beliefs :)
Alas, I could not find the origins of this.
They say it is impossible, because then the dream will not come true. At the same time, some believe that if you tell about a bad dream before 12 noon, then it will come true.