Continuity mistake: The tow truck and the red van it's towing change positions at the alley's entrance between shots. (00:01:00 - 00:23:35)
Continuity mistake: The wooden plank with which Alexandra impales Nick completely changes angles (from flat side tilted left to flat side tilted right) between the time it strikes him and the time he pulls it out. It also appears to "grow" an inch or so wider than it was before. (00:39:00 - 00:40:25)
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.