Crazy Love - S2-E26
Continuity mistake: The flashbacks here contradict series canon. Nick's victim, Amaya, is shown to have been bitten (by him) several times. In every other instance, unless he had multiple captive ladies to "sip" from, Nick's bite was fatal in one go. In fact, it was a major recurring plot point that Nick "couldn't stop."
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.