Though Emma discusses her reasons for never marrying in a couple of places throughout Austen's novel, a neat encapsulation of it occurs in Chapter 31, or Chapter XIII of Volume II. Emma is contemplating the various ways that Frank Churchill might propose to her following his departure from Highbury in response to a summons from the Churchills. The ball has to be cancelled. Emma is bereft of the pleasant attentions of the handsome and charming Frank. Most of all, she is certain he was on the verge of declaring his devotion when Mr. Weston came to the door to fetch him back home for a speedy return to the Churchills.
In Chapter 30, Emma's response to Frank's first try at revealing his feelings,
"I think you can hardly be quite without suspicion"—He looked at her, .... It seemed like the forerunner of something absolutely serious ....
is to deflect the topic by confirming that he did right to visit the Bates' household before coming to Hartfield. Though she intends to refuse Frank, she is warmed by the thought that she is a little bit in love with him.
And the reason she reiterates for her intention to refuse him is that of her determination to never leave her father. This core determination is augmented by her belief that a young woman in her position only restricts her options in life by marrying. She has independent wealth. She has a home for which she has been chiefly responsible and at which she has been hostess since the death of her mother.
She has the highest social position and influence in town. She is the chief benefactress of mercy and aid in the village. She has everything that contributes to comfort and position. There is no reason for her to marry and trade all she has for something perhaps lesser. Notwithstanding the above, her primary reason for not marrying is that she is devoted to her father and will not leave him:
it struck her that she could not be very much in love; for in spite of her previous and fixed determination never to quit her father, never to marry, a strong attachment certainly must produce more of a struggle than she could foresee in her own feelings.