Death be not proud.
"In the real world things are very different. You just need to look around you. Nobody wants to die that way. People die of disease and accident. Death comes suddenly and there is no notion of good or bad. It leaves, not a dramatic feeling but great emptiness. When you lose someone you loved very much you feel this big empty space and think, 'If I had known this was coming I would have done things differently.' These are the feelings I wanted to arouse in the players with Aerith's death relatively early in the game. Feelings of reality and not Hollywood."
-- Yoshinori Kitase, Edge Magazine, May 2003
For the two characters, Zack and Aerith, death was inevitable. Some say that their deaths came far too early, but their tragedy becomes their strength, and helps define Cloud's character in the rest of the seventh installment. Yoshinori expresses his views that death comes to us naturally, not for a specific reason or for some higher purpose. Though both Aerith and Zack may have died to save one particular person, the life that they save transcends the general greater good; they died protecting Cloud, because they believed that they could-- they chose to protect him, in spite of all odds. However, though both of them, as individuals, made that decision, the result is their death—neither one of them choosing nor planning it.
Throughout Advent Children, Cloud struggles to find a sense of closure for the loss he suffered. It's not just Aerith who's gone; it's Zack too. Cloud carries the burden of believing in the sacrifices made so that he alone may live. Thus, in the events preceding Advent Children, he succumbs to depression due to his inability to cope with their deaths. In the end, however, he learns that Zack and Aerith accepted their fate of sacrificing themselves not so that he could live for them in return, but so that he could live for himself.
For Zack and Aerith, it's quite ironic that the two characters can be torn apart, as well as brought together in the event of death. In one instance, their meeting in the lifestream brings a revelation to the condition of both characters. In another, one of their most heartfelt scenes displayed in the game is when Aerith comes to comfort Zack by embracing him. Zack and Aerith were left alone in the church shortly after the death of Angeal. While suffering from the loss of his mentor and fatherly figure, Zack lets go of his pride for an instant, and breaks down. Turning around, Aerith walks over to quietly comfort him; her embrace and his acceptance of it allows us to understand who they are in their most honest moments.