In the episode where Aru ask if Mukhi will not share the contents of the diary because if he thinks of her as wife or not, we missed one more important dialogue.
When he is hiding the diary and Aru is teasing Mukhi, She says" Mukhi ji, Aap to interesting hote ja rahe hai"
I guess a part of falling in love is this too. At least a part of initial attraction for a smart mind is not always physical but also the mental stimulation.
This probably sums up the attraction that Aru has started feeling for her rustic looking, much older than her, set in his ways, and from a not so happening place husband.
She also says in one of her dialogue before the book snatching scene, that this village is too not so quite as she assumed. (That one reminded me of Agatha Christie's Miss Marple and her village)
Someone who is smart, intelligent and curious like Aru may not get attracted by only physical attributes, and kindness builds fondness or affection. But love is a heady mixture of all of these ingredients. Affection, attraction, pull, devotion and a lot of other named and unnamed emotions!
Realizing Mukhi is not a one note stereotype does make a difference for her (and same for him too).