Blood Money - S2-E17
Trivia: Series soundtrack composer Fred Mollin appears behind Nick and Janette in a nightclub scene here, but didn't know he'd been set up for a practical joke when a sparsely clad female extra began passionately kissing him. The kiss became so personal, in fact, that it had to be removed from the final cut. Fred & friend are still in the scene, albeit sans kiss. (00:16:15)
Answer: Nick was sick and tired of being an immortal bloodsucker. He wanted to be human, fall in love, get married, have children, grow old and die. As for Janette, according to her, she fell in love and the passion she felt "cured" her of her blood lust.