Waise Arnav generally wasn’t a poor judge of character. I think Khushi affected him in a way; and deep inside he always knew it. His love for his mother and the betrayal she faced had devastated him.
He was always afraid of letting her get too close; and once he did give in to his feelings. He saw Shyam and Khushi together. What he witnesses is betrayal in two relationships - Anjali & Shyam; between him and Khushi. Not only that, he sees them both betraying his whole family. His trust in love is once again shattered.
Even though Shyam was harassing her, the terrace scene did confirm that this obviously wasn’t the first conversation between Shyam & Khushi. Khushi still hid the information from him; Arnav still would have expected her to tell him everything and keeping it a secret is a betrayal in itself from his point of view (Aakash was I feel, rightly angry with Payal.. but he dragged it too long). When someone you are emotionally involved with, hides something from you or lies about certain things, it causes trusts issues. (Something I can totally relate with. Thodi Arnav jaisi hun😆).
If it were for someone else, he may have given them a chance to explain. He confronted Shyam but he never confronted Khushi. He just didn’t want to listen to her. He was afraid of giving in and being heart broken yet again. Because even if Khushi did love Shyam, it would have been impossible for him to back off once he gave in.
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