My guess would be that he mostly met with more sophisticated women, who either hide their partiality to look unattainable or exaggerate their feelings to secure his affections - a la Isabella Thorpe (who apparently follows Charlotte Lucas advice LOL).

Catherine shows a sincere and unaffected affection for him, which probably made enough of an impression on Henry to put him on the path of love. I suspect that he was also flattered by Catherine's trust in his opinion and willingness to be taught by him. The narrator made some remarks on this very subject on Chapter 14:

"Where people wish to attach, they should always be ignorant. To come with a well–informed mind is to come with an inability of administering to the vanity of others, which a sensible person would always wish to avoid."

"[...] a good–looking girl, with an affectionate heart and a very ignorant mind, cannot fail of attracting a clever young man, unless circumstances are particularly untoward"