As voldemort shared some of is power with harry, they are connected in some way...for example when their wands acted strangely during their duel in the fourth book.now seeing as voldemort never actually dies and only like 3/4 of his power is destroyed wen he is defeated...this cud mean that some of his strength lies within harry, so as long as harry is alive, so is voldemort, no matter how weak he is...harry realises this and sacrifices himself to finish voldemort off.........what ya think?!!!