Pip finds Estella impossible to resist; she's like his own personal obsession. He sees her faults and sees her for who she truly is, but the passion holds him even when he knows better.
The unqualified truth is, that when I loved Estella with the love of a man, I loved her simply because I found her irresistible. Once for all; I knew to my sorrow, often and often, if not always, that I loved her against reason, against promise, against peace, against hope, against happiness, against all discouragement that could be. Once for all; I loved her nonetheless because I knew it, and it had no more influence in restraining me, than if I had devoutly believed her to be human perfection. (Chapter 29)