beautifully explainedWell, I think in case of Nandini, initially it was respect & admiration for Kunal for the person he is. Then, kunal literally appeard like her knight in shining armour protecting and rescuing her from every difficult situation. All these together lead to development of some feelings for kunal in Nandini's heart. These feelings might be termed as love but I think still superficial.
In case of Kunal, It initially started as a sudden intense attraction for Nandini, which literally shook kunal to his core, not only because it is wrong but also because, he was so sure about his love for Mauli that he used to believe that nothing can change his love for Mauli. In all these years of their married life, it not not possible that kunal has not come across prettier ladies than mauli. But certainly those ladies didn't attract kunal. But all of a sudden one day kunal found himself being attracted to Nandini to such an extent that his thoughts were almost always were occupied with Nandini. He tried fighting this attraction by avoiding Nandini and infact suggested that Nandini should move to her own house so that he can get over this attraction. But when Nandini herself decided to go to Rajdeep, at that moment kunal realised his magnitude of feelings for Nandini. In that scene kunal appeard as a person whose life is slipping from his hands but he is helpless and is unable to do anything about it. I guess, at that moment kunal realised that this is not mere attraction but much deeper feelings than that. At that moment he realised that fighting those feelings mean fighting an ever losing battle. So we see in his monologue in the next episode that he has come to terms with the feelings, he has understood that he can't control this feeling & so he accepts this. Now currently kunal thinks about Nandini's happiness first than anyone else and even he put Nandini's happiness before his happiness. This means now kunal is actually in love with Nandini & no longer in love with Mauli. It's very sad and unfair to Mauli but unfortunately it's the bitter truth ðŸ˜