Pip, having grown up in rather humble circumstances with a rough sister as a mother and working-class surroundings, is dazzled by Estella's beauty and grand circumstances. She is a beautiful young girl who symbolizes a life of education, money, ease and social contentment that he didn't even realize existed. She seems like she is completely out of his world, living in a world that is completely charmed and unattainable. Her comment to him about his rough, dirty hands only solidifies her essence of unattainablity; because of the seemingly huge gap in their statuses, Pip feels, for the first time in his short life, that there is more out there in the world. He first falls for her because she represents everything that his life is not that he wants it to become. She is an end-goal; if he can be worthy of her love, then he will have made his place in life. So, she is less of a real person for him, and more of a dream, a goal to be achieved.
Pip's love for her remains steady despite the fact that he knows she despitefully uses men and has a cold heart. Why? Well, he knows how she was raised. He knows Miss Havisham, and from the very beginning, was aware of Miss Havisham's rather insidious and pernicious influence on Estella and her perceptions. He knew that Miss Havisham was breeding Estella to break men's hearts, and to steel her own heart from the type of heartbreak that she herself endured. Because he knows Estella's background, it allows for understanding. He also spent so much time with her that he knows that isn't all that she is. He can see her potential, and have pity that she had such an unfortunate mentor fostering her emotional needs. He is attracted to her potential, and to her sub-obvious need for true, real love in her life. Plus, the old, strong pull of childhood fascination of her still holds sway in his heart, long after he realizes that she has other plans in life.
I hope that those thoughts helped; good luck!