Why do you think the narrator gives up his old life, and why does he pretend to be the professor?

The narrator is very depressed. He talks about trying to call a woman, so it seems clear that his relationship has recently ended. He seems to feel that he has nothing else to live for, and his old life suddenly seems unimportant. He decides to give up on it and just keep driving. At first he just continues on, not really caring about anything. The professor pulls him out of this reverie by talking about something interesting, and then just disappearing, leaving him with a ready-made life to step into. The narrator is sick of his old identity and seems incapable of forming a new one. It is easier to just pretend to be the professor. Soon he finds that he likes this, because he gets respect from others and can interact with people again. The pretense becomes part of him, and he actually adopts the professor's old life. This gives him a sense of purpose, though a part of him seems to be aware that this is false and not really fulfilling.