Blind Faith - S3-E5
Plot hole: In a major violation of series canon, this episode ends with the vampire doggie (a silly enough premise to begin with) bringing its owner across, a process previously established as complex enough to be far beyond even the smartest canine's capabilities. (00:40:00)
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.