A More Permanent Hell - S2-E20
Factual error: In the first scene, Dr. Dana says the asteroid heading for Earth is 5 miles wide, "larger than the one that caused the dinosaur extinction." The asteroid that did in the dinos is estimated by most scientists to have been 6-8 miles wide. (00:03:00)
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.