Even though Doetoevsky's underground man perceives himself as a deep, conscious, brilliant man, he still knows that he is skeptical of every thought or feeling he might have. He tries to convince himself of being smarter than any body he encounters, but in reality he has a deep feeling of inferiority that ultimately manages to isolate him from people and society.
The underground man never had any experiences of love or emotional relationship, thus he relies in his youth on literature and drama where he gets high expectations of ideal relationships and morals. However, real life interactions and relationships traumatize him with reality that he doesn't know how to accept.
In his forties, the underground man doesn't crave human interactions or attention any more, or have passionate ideas about any thing like he did in his youth, and he knows no other way than anger and bitterness to deal with people. Even though his intimidating way of dealing with people brings him humiliation and pain, he still enjoys thinking that he is practicing his free will. Ironically, the humiliation he brings down on himself is empowering and satisfying to the underground man. As long as he has choice and free will, he is still alive and active like others, regardless of the consequences of the choices he makes.
Whether Dostoevsky wrote notes from Underground as a scream against rationalism and utopianism, or if he was symbolizing his own alienation from the modern Russian society, he just did a great job. Every detail in the book is worth reading.