Continuity mistake: Twice in this episode, Janette refers to having been a vampire for 800 years. Previous episodes established that while Nick was 800 years old, Janette was 1000. (00:14:45 - 00:33:00)
Plot hole: Throughout the first two seasons, Nick was always able to sense other vampires and to instantly differentiate between vampires and humans. Here, he inexplicably fails to detect that the returned Janette is now mortal. She has to tell him. (00:27:10)
Revealing mistake: When Robert is shot, the car blowing up in the alley is sort of half an explosion, abruptly curtailed. Severely cold weather during shooting literally froze the cameras and left only a few seconds of usable footage. That snippet is repeated for effect, but the explosion still looks "cut off" because it is. (00:32:00)
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.