Father's Day - S2-E7
Continuity mistake: The godfather's thugs storm Nick's loft and riddle him with bullets. We actually see the bullets coming out of his back, leaving holes in the back of the vest he wears. But when the "vamped out" Nick turns around to face the camera, there are no bullet holes in the front of his vest. (00:23:20)
Father's Day - S2-E7
Continuity mistake: During his fight with Constantine's henchmen, the bullet holes in the back of Nick's vest disappear and reappear several times between shots. (00:23:30)
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.