"No, it is not life that I regret, but the ruin of my plans, which were so long in devising and so laborious to construct. Providence, which I thought favoured them, was apparently against them. God did not want them to come to fruition!
"This burden which I took on, almost as heavy as a world, and which I thought I could carry to the end, was measured according to my desire and not my strength. I shall have to put it down when my task is barely half completed. Ah, I shall have to become a fatalist, after fourteen years of despair and ten years of hope had made me a believe me a believer in Providence!" (90.2-3)
In order to believe he is an agent of Providence, the Count must believe he knows what is meant to be. To see that certainty crumble is, potentially, just as damaging as that fateful bolt from the blue, the stroke.
"What is truly desirable? A possession that we cannot have. So, my life is devoted to seeing things that I cannot understand and obtaining things that are impossible to have. I succeed by two means: money and will. I am as persevering in the pursuit of my whims as, for example, you are, Monsieur Danglars, in building a railway; or you, Monsieur de Villefort, in condemning a man to death; or you, Monsieur Debray, pacifying a kingdom; you, Monsieur de Château-Renaud, in finding favour with a woman; or you, Monsieur Morrel, in breaking a horse that no one else can ride." (63.7)
Unlike the others, who seem to do things because they love them or what they lead to – for instance, railroads lead to money – Monte Cristo claims to do things for the sake of, well, doing things, like a climber who scales a mountain because it's there.
"Poor young man!" Monte Cristo muttered, so low that even he could not hear these words of compassion as he spoke them. "It is written that the sins of the father shall be visited on the sons, even to the third and forth generation." (85.135)
Because sin itself is able to persist, according to Monte Cristo, and corrupt even the sons of those who sinned, the Count himself must persist in his pursuit of those sinners-by-proxy if he hopes to complete his job.