Simply put: The reader thinks of Humbert as a good, but tragic figure, because Humbert says he is. The reader readily buys into Humbert's idea of himself even though all evidence proves the opposite. However, one cannot continuously cause harm to someone with little to no regard for the person being injured, but call oneself a good person [on the inside]. But again, the irony is that the reader accepts his assertions, because he tells us to. He likes girls between the ages of 9-14. He's behaved badly well before meeting Dolores and her mother, but poof! It's forgotten before the paragraph ends. He continuously pokes fun at US citizens and thinks himself above them, but no one notices. He has a particular disdain for US pop-culture, but uses it to seduce the reader and Dolores.
We're made to believe that Humbert's story is one of love, but based on what? We know from the opening lines how Humbert found himself, so the story described is clearly being used to gain sympathy. The reason we know this is because throughout his re-telling, there are certain moments where he can be nothing but brutally honest about his behavior towards his Lolita. It should quickly become clear to any reader that this isn't a story of love. Humbert loves the idea of Dolores; she's Lolita only to him and him alone. He believes that the girl(s) he fantasizes about want him and are seducing him. So, they are asking for it--reminds me of something, I just can't think of what.
We, the reader, are asked to excuse Humbert and compliment him on his level of restraint, and most readers do just that! Humbert is able to show how a lot of people will ignore and/or make excuses for someone's bad behavior if they view the person as attractive in some way. As the reader accepts more and more of Humbert's tale, due to his enthralling voice, Dolores fades into the background. Instead of seeing Dolores as a 12-year-old girl who is exploring and mimicking sexuality and adult ideas, we allow ourselves to see her only as Lolita: a fully functioning sexual being who knows all there is to know about tempting and pleasing men.
This is why I love the book so much. Nabokov was able to go inside the mind of persons like Humbert and show how they twist reality to make them look more sympathetic.