Aeris is one person who would never get together with Cloud. She's the one who senses his fractured psyche. She doesn't know Cloud. At all. She tries to draw him out of his Zack persona with niggling questions about his past and his thoughts. What little attraction she has to Cloud is not to Cloud at all, but to the shadow of Zack that he is presenting to her. She, more than any other member of the cast, senses just how broken Cloud is. She spends the entire game trying to fix him, until she leaves him to fix the world instead.

Tifa actually knows Cloud, the real Cloud, the Cloud Aeris never got to see. She has a much more wholesome relationship with Cloud than Aeris does. And she cares for Cloud more than anything. Aeris abandoned Cloud to save the world. Tifa abandoned the world to save Cloud.