I think by suggesting Catherine should not dream of Mr. Tilney till she is sure he has dreamed of her the narrator is poking gentle fun at Catherine’s “strong inclination for continuing her acquaintance with Mr. Tilney” from Ch. 3. It is a fanciful and amusing constriction to place on Catherine however I think the original idea, that a lady should not let herself love a gentleman before he declares love for her, was accepted wisdom meant to protect young ladies. See Adrian’s post about the celebrated writer and original quotation linked below.

One need only look to other JA novels for young women made miserable because they loved before the gentleman declared his love for her. Jane Bennet and Marianne Dashwood both suffer for this reason. In P&P, Jane suffers misery of the acutest kind, doubts her own judgment and must bear the censure of the world for disappointed hopes after Bingley failed to return to Netherfield and propose. In S&S Marianne suffers much the same over Willoughby only she has her own behavior to regret as well. Mrs. Gardiner advised Lizzy not to let her fancy run away with her about Wickham in Ch. 26 of P&P and in S&S Elinor tried to control her feelings for Edward at first because he made no declaration and later because he was unavailable. So far Catherine seems to be following in Elinor’s footsteps. After one meeting Catherine desires to know Henry better but she shows no signs of letting her fancy run away with her yet. In Ch. 6 she tells Isabella: “But you should not persuade me that I think so very much about Mr. Tilney, for perhaps I may never see him again.” Sensible girl!