Kunal's feelings for Nandini does come across as random.
But if you have observed, makers have deliberately shown Mauli and Nandini with polar opposite personalities. Mauli is practical, whereas Nandini is emotional, Mauli is bubbly and outgoing, while Nandini is silent and soft-spoken. Nandini is also traditional, which Mauli is not. Nandini also has this sensitivity towards other people's feelings, and Mauli herself keeps saying that Kunal would like Nanidni's choices and would have liked Nandini too, had he met her first. So somewhere they are trying to tell us that Kunal finds a connect with her as a person, but with them rushing through that phase they have failed to impress it upon us more effectively.
Plus, the butchering to Kunal's character is off-putting. The guy had one night of guilt and now nothing. But I know he will come back to reality soon.
