"Possibly," you will add on your own account with a grin, "people will not understand it either who have never received a slap in the face," and in that way you will politely hint to me that I, too, perhaps, have had the experience of a slap in the face in my life, and so I speak as one who knows. I bet that you are thinking that. But set your minds at rest, gentlemen, I have not received a slap in the face (1.3.3)
Either the Underground Man is speaking literally and has never been challenged to a duel, or he is speaking metaphorically and has never truly been offended. The latter carries much weightier implications; it would suggest that he knows his attempts at revenge have been pointless and irrational.
Of course I have myself made up all the things you say. That, too, is from underground. I have been for forty years listening to you through a crack under the floor. I have invented them myself, there was nothing else I could invent. It is no wonder that I have learned it by heart and it has taken a literary form....
I write only for myself, and I wish to declare once and for all that if I write as though I were addressing readers, that is simply because it is easier for me to write in that form. It is a form, an empty form – I shall never have readers. (1.11.5-7)
The Underground Man is so isolated that even these Notes – his one opportunity to communicate with the world from underground – are a solitary endeavor.
I did not, of course, maintain friendly relations with my comrades and soon was at loggerheads with them, and in my youth and inexperience I even gave up bowing to them, as though I had cut off all relations. That, however, only happened to me once. As a rule, I was always alone. (2.1.9)
The Underground Man makes it clear that his flight to the underground was a long time coming. At no point in his life has he ever been at home in society. This definitely affects the way we look at his character. Rather than trying to find some reason for his isolation, some specific event, we attribute his solitary confinement to his unchanging character.
One night as I was passing a tavern I saw through a lighted window some gentlemen fighting with billiard cues, and saw one of them thrown out of the window. At other times I should have felt very much disgusted, but I was in such a mood at the time, that I actually envied the gentleman thrown out of the window – and I envied him so much that I even went into the tavern and into the billiard-room. "Perhaps," I thought, "I'll have a fight, too, and they'll throw me out of the window." (2.1.12)
The Underground Man is so starved for human contact that even getting beaten up appeals to him; at least then he would be noticed and recognized as a man.
I could never stand more than three months of dreaming at a time without feeling an irresistible desire to plunge into society. To plunge into society meant to visit my superior at the office, Anton Antonitch Syetotchkin […who] usually sat in his study on a leather couch in front of the table with some grey-headed gentleman […]. I had the patience to sit like a fool beside these people for four hours […]. I was overcome by a sort of paralysis; but this was pleasant and good for me. On returning home I deferred for a time my desire to embrace all mankind. (2.2.3-4)
Try as he might to convince himself that life underground is the way to go, the Underground Man is clearly dissatisfied with his solitary life.
My schoolfellows met me with spiteful and merciless jibes because I was not like any of them. But I could not endure their taunts; I could not give in to them with the ignoble readiness with which they gave in to one another. I hated them from the first, and shut myself away from everyone in timid, wounded and disproportionate pride. […] In the end I could not put up with it: with years a craving for society, for friends, developed in me. I attempted to get on friendly terms with some of my schoolfellows; but somehow or other my intimacy with them was always strained and soon ended of itself. (2.3.39)
This conflict, present in the 24-year-old Underground Man, remains a part of the 40-year-old Underground Man. While the (older) narrator claims that he doesn't have readers, his Notes are nonetheless an attempt to reach out to someone else (even an imagined someone else).
Once, indeed, I did have a friend. But I was already a tyrant at heart; I wanted to exercise unbounded sway over him; I tried to instill into him a contempt for his surroundings; I required of him a disdainful and complete break with those surroundings. I frightened him with my passionate affection; I reduced him to tears, to hysterics. He was a simple and devoted soul; but when he devoted himself to me entirely I began to hate him immediately and repulsed him – as though all I needed him for was to win a victory over him, to subjugate him and nothing else. But I could not subjugate all of them; my friend was not at all like them either, he was, in fact, a rare exception. (2.3.39)
The Underground Man's story about the "friend" he subjugated in his youth foreshadows his interaction with Liza, when he tries to do the same thing to her.
And how can it all be done by daybreak? and where am I to get a second? I have no friends. Nonsense!" I cried, lashing myself up more and more. "It's of no consequence! the first person I meet in the street is bound to be my second, just as he would be bound to pull a drowning man out of water. The most eccentric things may happen. Even if I were to ask the director himself to be my second tomorrow, he would be bound to consent, if only from a feeling of chivalry, and to keep the secret! Anton Antonitch... ." (2.5.11)
The Underground Man still has no appreciation for the difficulty of breaking out of isolation.
It was the exercise of my power that attracted me most. (2.6.90)
The only way the Underground Man knows how to interact with others is by inflicting pain on them or by showing his superiority.
[…] with me loving meant tyrannising and showing my moral superiority. […] Love really consists in the right – freely given by the beloved object – to tyrannise over her. (2.10.2)
"Liza!" I cried, more loudly.
No answer. But at that minute I heard the stiff outer glass door open heavily with a creak and slam violently; the sound echoed up the stairs.
She had gone. I went back to my room in hesitation. I felt horribly oppressed. (2.10.11-3)
Notice that the Underground Man feels "oppressed" both before and after Liza leaves.