Why the lies?

Up until this point, I have believed everything the Underground man has said. Like I said in my last post, everything he says is believable and relatable. Also, why would he lie about what he is saying? What possible reason could there be? When he states that everything he said in the past chapters was just a "bitter joke" I was completely shocked. I honestly did not know how to process this news because. What is it about this man that he feels the need to create these stories and lies just to proceed to destroy them? I really liked him before but now I am not so sure. The whole book has been a lie...unless he is lying about lying, which I suppose could be a possibility. The narrator is playing mind games with us and I do not know how I feel about it.