Continuity mistake: The actor playing the bad guy apparently didn't stick to the script. What he says on the phone, telling Tracy where to meet him, and what she writes down for Nick to find later, don't match at all. (00:35:00)
Continuity mistake: The blood stain on Tracy's lapel changes its shape several times between takes. (00:08:15)
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.