Sadhana, they are right. Read my reply carefully, you stated his self-loathing as the one reason for his behaviour but that's not it.
His mother has contributed a LOT to his self loathing, and when a person hates himself/herself, they tend to look for someone who they think are "perfect".
Remember his lines to Dhruv after the storeroom episodes? "Mera kya hain...I like to be BAD".
Remember his lines to Nandu when he was drunk? "Main hamesha itni saari galtiyaan karta rehta hoon."
Dhruv used to be the ideal one in his eyes. The one who can never go wrong. So when he got to know that he has kept mum on the goons incident- that too in front of Harshad- he was rendered heartbroken.
Similarly he put Nandini on a pedestal in his mind because not only he saw goodness inside her, he saw a person who had the capability to love someone unconditionally! He saw someone who gifted him a lamp with star-like designs on it's body despite hating the stars herself- just because HE likes them. He saw a girl who kept her in her prayers without him asking her to. He saw a girl who promised him so much without him confessing his love to her.
And when he heard the same girl doubting him- and that too for a crime so huge- even if for a few seconds came as a BLOW to him!