Love Hurts is a short story in The Dresden Files series by Jim Butcher. It takes place between Turn Coat and Changes. It was originally published in the Songs of Love and Death anthology, and later also included in Side Jobs.
Karrin Murphy and Harry Dresden are investigating a double suicide in an apartment in Wrigleyville involving a brother and sister who seem to be a couple. They have been mentally manipulated to fall in love with each other, and became insane; it's the third couple in a month.
Dresden briefly contacts Anastasia Luccio, who has no clue about the events: Dresden fails to come up. He spends four days combing the supernatural scene for any information, turning up zilch. Murphy, however, finds the common denominator between the victims: the Illinois State Fair in Springfield. The two of them drive down there to check it out.
At the fair, they're tailed until they arrive at an attraction called "TUNNEL OF TERROR", but the tailer has disappeared; they bribe the carnie to enter the attraction from the back door. Adding giddy and unusual, and they realize they're subject to a love spell. Getting back to the carnie, they interrogate him, harshly, and he leads them to a ladder that leads to a secret room, where they find the guy who tailed tham and his employer; Baroness Leblanc of the Red Court. She is attempting to create true love in people, so that she can both weaken her competitors in the White Court and have a higher population to feed upon.
In the ensuing confrontation, the Baroness taunts Dresden, causing Murphy to attack, shooting the Baroness dead. The vampiress is no practitioner and could not have been casting the love spell; nor could her thrall. Dresden and Murphy connect the seat belts of the attraction with the FBI wolf belt used in Fool Moon and burn them, destroying the spell.