I decided to copy and paste this essay from Gospel Angel. I'm really sick of having to retype essays on the same topics, so yeah..
I do not agree with the Zack x Aerith pairing. I was never fond of the idea, and I doubt I'll ever be. I understand why people support the coupling, but the storyline of their relationship was too vague to even care about. It just seemed unimportant. Not that Aerith and Zack weren't important as separate characters, but their relationship just reminded me of immature highschoolers. Now and then Aerith talked about Zack, and there were a few flashbacks, although there were not any flashbacks of Zack and Aerith together. So that makes it even harder to figure out what really happened between them.
Since their relationship didn't go into complete detail, I believe Square intended that. It wasn't very important, so they didn't give a lot of attention to them. I don't think Zack and Aerith were ever inlove. They were just teenagers at the time. As Aerith said so herself, their relationship wasn't serious. Yes, she does bring him up. But seeing that he all of the sudden disappeared, why wouldn't she wonder about him? Since they weren't all that serious, I do believe Aerith wanted it to be. And that maybe she wanted to get to know him better. And that would explain why she wanted to get closer to Cloud. So that maybe he would replace Zack, and in her eyes, she would be able to finish the relationship with Cloud, that she was never able to with Zack. But in the process, she fell for Cloud and chose to let go of Zack.
But even after all the time that Aerith and Cloud spend together, if Zack were to have lived and shown up, Aerith would probably still choose Cloud. And a lot of people bring up the deaths of Zack and Aerith, and that maybe they could be together in the afterlife or the Lifestream. Which is a good theory, but like I said. They weren't even really inlove. They were just teenagers at the time who barely knew eachother.