I think it bothers Sai a lot that he married her even though he has stated that he cannot give her the rights of a wife and that he wont openly acknowledge that he loves paakhi and keeps referring to her as best friend! Its a little ridiculous.
Also i wouldn't be surprised if apart from her aspiration of becoming a doctor she also aspired for a love marriage and a partner who had her dad's ideals....she has found someone similar in virat. But problem is he only sees her as a responsibility and that definitely irks her. But she didnt really get too much choice. She has no family left, even pre-virat, she intended on moving to nagpur so marrying him is her ticket out of the village and on the path to achieving her dreams. Since her father held virat in such regard and everyone around pushed her, she eventually agreed to a contract. I dont think as an 18 year old she could have given much thought to the fact that marriage cannot just be a contract especially when she is stuck in this large joint family with far too much drama !