While Persuasion is on one level a romance, it also presents some very unromantic ideas about marriage. Sure, it would be great if you could spot your true love a mile away, get hitched, and live happily ever after. It's more likely, however, that two people will meet, get involved, and then realize that the beloved is not all he or she seemed to be. The lucky characters manage to figure that out before they get too serious, and the even luckier ones end up with a happy marriage anyway, but it's Chance and not Cupid that's running the marriage game.
The novel's opposition to long engagements suggests that it's impossible to be certain of the character of one's spouse before marrying them, and therefore that wedded happiness is up to chance.
The novel's opposition to long engagements suggests that the choice of a spouse should be based on gut instinct rather than thoughtful judgment.