And yes, at the beginning Aeris does like Cloud because he resembles Zack. But who wouldn't? If I lost somebody and then saw the qualities I loved about them in another person, I'd take a liking to that person to. After being alone for such a long time, people tend to seek companionship. After all, why do people remarry? She sees this knight in shining armor that literally falls into her lap and reminds her of her lost love. What do you expect her to do? It is also made clear that Aeris changes and develops into a stronger person, loving Cloud for being Cloud in that gold saucer scene "Cloud I want to meet you."