Factual error: The costumes in the flashback to Nick's wedding are a mishmash of the mismatched, coming from several different eras, including a full metal suit of armor that's at least 2 centuries ahead of the flashback's period setting in the 1200s. (00:14:00)
Factual error: In the flashback to Nick's wedding night, his medieval tunic/shirt has 20th Century white plastic 4-hole buttons sewn to the cuffs. (00:29:25)
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.