Because help exposes him and puts him in debt, and he knows how to relate to others only through power. The narrator gives the explanation himself in Part Two: "for me to love meant to tyrannize and to be morally superior."
The case is that of Liza. When she embraces him, in Chapter IX, he cries genuinely, but, still face down on the sofa, he feels the roles have reversed: the humiliated one is now he. Lifting his head, the shame turns into something else, and the text says that he hated her and desired her at the same time, in a feeling that was almost a revenge. He confesses that without power over someone he cannot live.
The cause is wounded vanity. He tells Liza that he is more ashamed than if he had stolen, "because I am vain as if my skin had been torn off, and even the air hurts me." Being seen weighs more for him than being guilty, and whoever touches him wounds him.
The pattern repeats. At school he had a friend: "when he gave himself to me entirely, I immediately hated him and pushed him away, as though I had needed him only to win a victory over him, only for his submission." A completed victory takes away the object. The money he puts into Liza's hand, "out of spite," is the last blow. The text leaves two readings open: a confession of an incapacity, or an explanation that excuses him from what he did.