I see Catherine's growing aversion to John Thorpe as being conditioned about equally by JT's loutish and tedious behaviour, and by the fact that he gets in the way of her developing her acquaintance with Mr Tilney, whom she much prefers.

At the ball in Ch.8 JT shows up late for their first dance, fails to apologise when he does turn up, then treats her to boring conversation about horses and dogs. Also, CM is “disappointed and vexed” because she has been obliged to decline Mr Tilney's invitation to dance – hence her refusal to dance again or walk about the room with JT.

In Ch.9 CM had planned to go to the Pump Room in hopes of meeting Miss Tilney (and perhaps her brother), but gets talked into going for a drive with JT instead. His company does not please her:-

“Little as Catherine was in the habit of judging for herself, and unfixed as were her general notions of what men ought to be, she could not entirely repress a doubt, while she bore with the effusions of his endless conceit, of his being altogether completely agreeable. It was a bold surmise, for he was Isabella’s brother; and she had been assured by James that his manners would recommend him to all her sex; but in spite of this, the extreme weariness of his company, which crept over her before they had been out an hour, and which continued unceasingly to increase till they stopped in Pulteney Street again, induced her, in some small degree, to resist such high authority, and to distrust his powers of giving universal pleasure.”

When Catherine learns that she has missed meeting the Tilneys at the Pump Room, her opinion of JT hardens:-

...she could only lament her ill luck, and think over what she had lost, till it was clear to her that the drive had by no means been very pleasant and that John Thorpe himself was quite disagreeable.

In Ch.10 Catherine wants to avoid JT not only because his company is wearisome to her, but because she wants to dance with Mr Tilney. She ...was now chiefly anxious to avoid his sight, lest he should engage her again; for though she could not, dared not expect that Mr. Tilney should ask her a third time to dance, her wishes, hopes, and plans all centred in nothing less.