Musar על מגילה 20:4

ויהי ביום השמיני . Our sages (Bereshit Rabbah 42,4) have taught us a rule that whenever the Torah uses the expression ויהי, this is a reference to a painful experience. The sages are sometimes hard-pressed to demonstrate the validity of this dictum. Our question is why the Torah chooses to use an expression which points at something unpleasant when we are taught that elsewhere (Genesis 7,2) the Torah spent additional verbiage such as הבהמה אשר לא טהורה, "a category of animal which is not ritually pure," instead of simply saying בהמה טמאה, "an impure animal," merely to avoid referring expressly to something unpleasant?