to me it seems as though he enjoys the frivolity of her character

i would have started counting from the beginning how many times he calls her "pretty"

i think he is interested in her because she is kind of a tease, and he finds that intriguing

he appears to believe that American women are completely different in manner than the foreign ones, and finds it refreshing

he acts as if he has never seen another girl like her before, or for that matter any other girl at all.

I think part of it may also be the puzzle. Throughout the story he's trying to figure out if she's a "nice girl" or not, innocent or not.

Some of his attraction to her, beyond her initial beauty, is her mysterious personality. She says things that catch him off guard.

I think he is just so used to the formality of European women, so when he comes across an American woman its new and exciting to him.

A mixture of her beauty, charm, and he believes that she is a good and innocent girl.

I think he is attracted to her impulsive behavior. He seemed to have been puzzled as far as how to respond too many of her actions.