In The Scarlet Letter, what does Chillingworth believe is the cause of Reverend Dimmesdale's problems?

In Chapter IV
whenvisits Hester at the prison, he asserts that he will learn who the man is that has committed
sin with Hester because

"[T]here is a sympathy that
will make me conscious of him. I shall see him tremble. I shall feel myself shudder, suddenly
and awares. Sooner or later, he must needs be mine."

This "sympathy" between Chillingworth andis in their pride. When he
identifies this pride in Dimmesdale, Chillingworth questions the minister and learns that
Dimmesdale feels that even a sinner can help his congregation. Thus, the old physician surmises
that Dimmesdale hides his sin of adultery in order to continue to minister to the members of his
church.

It seems that Roger Chillingworth suspects Reverend Dimmesdale from
the first when he stands in the crowd and watchesin her public humiliation. At the least, he
witnesses the minister encourage her to respond to the questions of the Reverend Wilson and
speak the name of the "fellow-sinner" who "hath not the courage to grasp it for
himself--the bitter, but wholesome cup that is now presented to thy lips!" As he says these
words, Dimmesdale's voice is broken and "tremulously sweet"; when Hester refuses, the
minister murmurs, "She will not speak!" and he has placed his hand upon his heart when
awaiting Hester's response.

The sympathy which Roger Chillingworth recognizes
in the Reverend Dimmesdale is that both of their good natures have been wasted. The physician
Chillingworth suffers as he has overstepped his human role as a man, and takes upon himself the
quest of a man's soul. Dimmesdale has also wasted his life in pretense; as a man of the cloth,
he has been hypocritical in preaching sermons against sin when he himself is guilty of the
serious sin of adultery. Thus, in his sin of pride, the physician for Dimmesdale feels the
sympathy of this same cardinal sin. For, in his pride, the minister has rationalized that
although a man be guilty of sin, he may yet retain "a zeal for God's glory and man's
welfare," and aware that if his sin were known he could achieve no good, he hides this sin
in order to help his fellow man and serve God. It is at this point that Roger Chillingworth
discovers the minister's chest holds the answer for him that he has suspected, "[W]ith what
a wild look of wonder, joy, and honor!" he gazes at the mark upon the minister's chest, the
reason for his guilt. Thus, Reverend Dimmesdale's secret sin of adultery is revealed with the
mark that the fiend who seeks revenge witnesses.