Continuity mistake: Nick gets a phone call telling him another murder victim has been found and that she's been dead for at least a week. In the very next scene, Schanke says that the victim "disappeared after a date last night." (00:27:00)
Factual error: In the flashback, when Nick regenerates on the autopsy table, his shredded clothes repair themselves, too. Maybe Nick buys his outfits in the same place the Incredible Hulk does.
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.