Why did aerith die

She died because Sephiroth killed her. Contrary to what some people still seem to believe, she did not sacrifice herself, she had no idea she was in any danger, and she planned to return to the party once she'd summoned Holy. It wasn't a total loss, though, since her death made it possible for her to utilize the Lifestream with the power of her will, which she probably wouldn't have been able to do while alive, meaning that the Planet just might have been destroyed had she not died.