A moral is a truth, message, or lesson. When a person reflects on something, they consider it carefully. In "The Gift of the Magi," O. Henry writes that Della's crying "...instigates the moral reflection that life is made up of sobs, sniffles, and smiles, with sniffles predominating."
A language that is more typical of modern times, the author might have said that Della's "howling" caused to her realize that life is made up of pain, sadness, and happiness, with the most common of the three being sadness. The author might also have simply said that moments of sadness are more common than moments of pain or happiness.