He would get into trouble simply because he was bored. In order to live life he had to invent some adventures for himself. He would take offense for no reason and even tried to fall in love once, actually feeling all the accompanying emotions, although he always knew that he was deceiving himself. As a result of boredom, the Underground Man was finally taken over by inertia, which involves doing nothing. He insists that inertia is the natural consequence of consciousness.
Spontaneous men can act only because they mistake secondary causes for primary ones.
One cannot act until one is convinced that one's action is right. In the case of revenge, for instance, spontaneous men are convinced that their revenge follows naturally from the cause of justice, and they are then satisfied that they can act. The Underground Man, however, insists that consciousness can see that justice is only a secondary cause. If one tries to understand the cause of justice, one will see that it is brought about only by other causes and those, in turn, result from other causes, and in the end every cause can be boiled down to the laws of nature, for which there is no one to blame and thus no one to seek revenge against. The Underground Man suggests that spite could replace the primary cause for action because it is not a cause at all, but he can't even believe in spite.