Why is Humbert Humbert not a reliable narrator in Lolita?

Humbert Humbert is a sociopathic pedophile who wants to justify his actions, which makes him an unreliable narrator. He would like to present himself as hopelessly romantically "in love" with Lolita, who is a prepubescent girl. He also presents her as coming on to him.
We have to remember that, unlike in the movie version, in the novel, Lolita is a little girl. She is not a teenager. She has no breasts, hips, nor adult sexual features. That is why Humbert is attracted to her. Her mother's breasts and other adult features repulse Humbert. Humbert worries about Lolita entering puberty, because then she will no longer attract him, and he will have to get rid of her.
Lolita does innocent things like sitting in his lap (he is, after all, her stepfather) that happen to sexually arouse Humbert, and this, in his mind, justifies later repeatedly raping her. He knows she hates it: he talks about listening to her cry herself to sleep at night in their motel rooms, and he has to give her increasingly larger gifts and become more and more insistent to get her to agree to do what he wants sexually.
If you actually "love" a person as Humbert claims to love Lolita, you don't rape her and do things that you know she hates to her. You put her interests ahead of your own. But Humbert shows himself to be a sociopath in his lack of empathy. He knows that she hates what he is doing to her, but he just doesn't care, as long as he can get her to come around enough to service him sexually.
Humbert tries to justify destroying the innocence of a young girl to indulge his own warped idea of pleasure. He uses beautiful language to try to justify using a child to satisfy his sexual desires. He uses deception repeatedly to get away with raping her and tries to deceive us into thinking this is love. He is not reliable, any more than any other sociopathic pedophile, and it's important to see past the self-serving story he tells to understand that he is a sick creature preying on a little girl.