It is widely believed that the Crestfallen Warrior is another Chosen Undead, one who fears losing his grasp on Humanity and becoming Hollow. Thus, he refuses to venture outside of Firelink Shrine. This is evidenced by his knowledge of the path up to the second Bell of Awakening. He has tried several times to ring the second Bell of Awakening and has also seen many other fail in the attempt. Thus, he claims that the path of Chosen Undead, the ringing of the second Bell of Awakening, is "impossible" or "suicide".
Then, when the player character rings the second Bell and awakens Kingseeker Frampt, the Crestfallen Warrior is struck with a sense of amazement. How could this person ring the "impossible" second Bell? Thus, he journeys to the New Londo Ruins and becomes Hollow. Here, his motivations are unclear. Was he simply twisted by jealousy and resentment and turned Hollow? Or was he inspired by your impossible achievement and made one last tragic attempt at completing his quest?