Aerith is the definition of a damsel in distress. As soon as she goes off on her own and does her own thing... well...
did you play the game or skim through it?
Aerith knew exactly what she was getting into, she needed to die to release HOLY and trigger the lifestream. Which is what happens in the end and why she's the last person we see (not counting Red XIII).
She left because CLOUD was the fukk-up who Sephiroth/Jenova could control.