@ All of you saying that Aerith wasn't falling for the Zack but actually parts of Cloud and so on and so forth...
There are tons of quotes saying she did just that, was falling for the Zack, saw the Zack in Cloud. If the original game didn't clue you in enough they repeat everything she does with Cloud with Zack in CC to show just how mirrored the two relationships were.
I'm sorry if you think it makes Aerith shallow and desperate, but she's not. Maybe a bit desperate to meet Zack again on the inside. It's not like she's going after Cloud completely consciously because of it. She's aware of the similarities, but remember she herself doesn't know that Cloud's actually messed up and actually acting exactly like Zack (or exactly like parts of Zack).
It's not just some guy who is kind of like him. This is a guy who's personality is being formed partially by the very man who she loved and who died. He falls through her roof, they go to the park, he has the same eyes, clothes, sword... no one can blame Aerith for wanting to find out the reason why, or for falling for him a bit.
I don't believe Aerith was actually 'in love' with Cloud though. Even ZaCloud. It's such a small amount of time they know each other and even then, there's a lot going on. Especially on her date. I always took her lines to be, "I fell kind of fast for you because you remind me so much of my old boyfriend, but now I'm ready to back up and try again looking at you for YOU, and not for him."
@Legretta: That's exactly how most of us feel. It doesn't make sense if that is what Aerith was saying, because they flat out weren't. Even if Cloud loved Aerith and Aerith loved Cloud, they are never shown as actual "lovers".