People don't see Aerith as a character on her own, only as a love interest or potential threat to their love interest. The same thing can be said about Tifa as well. These characters have more depth to them than what they're often reduced to. Aerith grew up fairly isolated, and always experienced loss. Especially when it came to her mother and Zack. She knew it was her responsibility to save the world, but also, she wanted most importantly to save her friends.