From the CV's POV, simply to stretch the matter cause had he told her, I don't think the marriage would have even happened cause Khushi would have told him the truth which she was anyway about to before he suddenly came up with his marriage idea.
Arnav wouldn't have been able to force her into the marriage after that because Khushi very much had evidences and witnesses (I guess) to prove it all (her family)
So the story would have taken a complete other turn which the CVs weren't able to handle because it's Indian show! An Indian show isn't complete without a forced marriage.
And secondly, because he feared her confessing it. He didn't want her to confess it and admit that she loves Shyam. He just couldn't hear it from her, he didn't want to be true. And I can completely understand that. Sometimes there are things you doubt or know which you don't want to be true, so you don't confront that person because until the person doesn't confess it, you can lie to yourself and try to calm down by saying maybe it isn't true. There's at least this 1% hope that it isn't true.
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