And what are you reading, Miss—?” “Oh! It is only a novel!” replies the young lady, while she lays down her book with affected indifference, or momentary shame. “It is only Cecilia, or Camilla, or Belinda”; or, in short, only some work in which the greatest powers of the mind are displayed, in which the most thorough knowledge of human nature, the happiest delineation of its varieties, the liveliest effusions of wit and humour, are conveyed to the world in the best-chosen language.
-- from Jane Austen's defense of the novel in Northanger Abbey
A 1799 book review in The Quarterly Review explains the biggest concern -- impressionable young people are so caught up in sentimental romance, that real life seems tame in comparison: “those romantic visions which throw into a dead gloom the brightest scenes of real life. And yet, “real life is the very thing which novels affect to imitate; and the young and inexperienced will sometimes be too ready to conceive that the picture is true, in those respects at least in which they wish it to be so. Hence both their temper, conduct and happiness may be materially injured."
Real life, even real love, and the realities of married life, are nothing like a sentimental novel. The Quarterly Review warns that novels teach susceptible young ladies to believe that “no sacrifice can be too great for real love; that real love such as subsists, and ever will subsist, between herself and the best of men, is adequate to fill every hour of her existence, and to supply the want of every other gratification, and every other employment.”