Arjun is so confused and dumb that he can't understand what he wants. Naintara at least knows that.
He can't even make sense of situation at hand.
Naintara told him that Dev is with Maan and Now he finds it so hard to understand something fishy in his DEAR Sister's thin explanation of events.
Shouldn't he think that why Maan is so angry with his brother when he has done exactly what his brother wanted? (according to Naintara: Dev gave her divorce because of Maan)