Tarrlok witnesses Korra's transformation during the course of the Equalist threat, during which they come to an agreement, one that's mutually beneficial—or so they’ll fool themselves into believing.
(Or—Korra, who possesses an incredibly deviant and voracious sexual appetite, fears commitment because she knows that her taste in men is terrible. Tarrlok, who hates himself, thinks he's in love with her. They struggle to meet somewhere in the middle.)