The Jack (or Joe from the book) is escaping reality and taking the easy way out of life when he joins the cancer groups as he tries to "hit rock bottom". Fight Club was also a means of escape and hitting rock bottom, and in the end he has to face Tyler and say no to him running Jack's life. The ending happens the way it does because Jack needs to face up to reality and make the changes in his life that he wants to happen - not waiting for someone else to come and change it for you.