The flinch on Sai's face was very obvious after Virat's dialogue 'Dosti ka farz'. That flinch was an indication of deep pain or unrequited emotional need. Here are my thoughts on the reasons (It is mixture of many emotions) behind that flinch:
1. She definitely feels the pain of being part of marriage that has come with an expiry date (it is not specific to Virat but marriage as an (sacred) institution. She has observed her Aaba's love for her Aai (he did not remarry). Psychologically we do have impact of these social notions (Even Pakhi's mom said today Shadi ke Sindoor ke age pyar ka gulal fika pad hi jata hai (her daughter is an exception to this though😆) I have observed this pain at wedding (mangalsutra scene- janam janam ka bandhan), immediately after wedding at Chavhan house ( she repeated to herself 'its a deal'), when Usha mavashi said'Unake sath ab janam bhar rehana hai'.
2. Virat is the only person after her Aaba who has not only shown her warmth but has taken her responsibility too. He fought tooth and nail to get her in the house, protected her when possible. For an 18 year old (with no father figure in life) it is very difficult not to get emotionally attached with such a gem of a person (Virat sir bahot hi nek insaan hai!) only to be thrown at with the word 'Farz' devoid of any emotion.
3.She herself had felt connection (even though a fragile one) with Virat in GC. 'Mai Virat sir ke bare me kyun soch rahi hu?'
4.She is attracted to Virat ( She did not mind Virat putting those hair strands behind her ear).Looking at her molestation history, this scene is very significant as people with such history are extremely sensitive to 'TOUCH'.
5.The most difficult situation in a relationship is when a person displays behaviours which indicate warmth, care (head massage) but cautions you, 'Hey I act the way I act but watch yourself your are not allowed to get emotionally involved with me' .I am just doing my 'Farz'.
Virat, we are humans and not computers to keep our brains and heart in seperate compartments. The word 'Farz' will come back to haunt you when Sai will interpret your love actions as your 'Farz'.
She already said multiple times today 'To mai apake liye sirf ek farz hun'
I would also like to mention@ janakiraghav's post on sense of belonging here. Sai is looking for sense of belonging too.
Would love to hear your perspective..!
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