Originally posted by: SheThought
In the beginning of the serial, when Mishti and Kuhu talked to Ketki about saying No to marry someone without knowing or meeting them first, Meenakshi (Big Ben / BB) heard it over the video and was very unhappy with the independent thinking and the advice given to Ketki. At first she concludes it was Kuhu who gave that advice, so when Rajvansh family takes Kunal rishta to Mahs, BB changes the proposal from Kuhu to Mishti. But afterwards she realizes it was Mishti behind that advice, but it was too late by then as the proposal was already in motion. Then Mishti keeps the condition of getting to know Kunal through the premarital courtship, so that makes her dislike Mishti even more because BB wants a bahu that can be seen but not heard. She realizes how Mishti is used to speaking her mind, how she goes against Kunal and says No for the lasik surgery and then when the time comes she even rejects Kunal and does not go through with the engagement. So all this cements Mishti's bold nature in BB's mind and in recent episodes she also learns of Abir being in love with Mishti. She is against Abir-Mishti relationship as she definitely does not want anyone that smart in Abir's life because then that person will not keep quiet if she sees something out of ordinary.
BB and Parul are hiding a secret that Kunal is actually Parul's son and somehow related to BB/RVfamily. She doesn't want the secret to come out or the brothers to know about it and wants the two brothers to always have a close bond. She sees Mishti as a threat in her so called mission of keeping the family together.
On the other side, Kunal has a giant ego and it got hurt when Mish rejected him by refusing to go ahead with the engagement. During MishKun courtship, BB insured that they have plenty of misunderstandings, so that combined with Kunal's big fat ego, he started to see Mishti as this manipulator who is not good for his family. He has also observed through many incidents how Mishti has a strong hold on his brother Abir, so he is also not favor of Mish-Abir union.
BB wants revenge from Mish and Maheshwari family for breaking Mishti-Kunal engagment and insulting Rajvansh family / hurting Kunal. Kunal and BB also want Abir to keep away from Mishti. So ma-beta deviced this plan of initiating Kunal-Kuhu (KuKu) wedding but not actually completing it. That way if they leave Kuhu at the altar that will bring shame and humiliation to the Mah family and BB's revenge will get fulfilled. At the same time, the shame to Mah family will also affect Abir-Mishti relation and that way they can keep Abir from "evil" Mishti.
They tried to have Mishti insult her own family and put a condition for Mish to stay away from KuKu wedding and thought Abir will start hating Mishti but obviously they underestimated Abir's love and trust on Mish. So in short, their whole plan has failed now and Kunal is compelled to marry Kuhu and MishBir are together.