I think that the reason why Yash is angry with Arti is because he probably read that damn DIARY!!
Remember Arti's diary, which she was trying to hide from Yash...In which she had written all her feelings...
I'm damn sure Yash caught hold of that diary...Now that he knows that Arti has feelings for him, he's trying to make sure she starts hating him...He doesn't want her to like him. He thinks, by being mean and cruel...she'll be disgusted at his behavior and dislike him...
He probably doesn't want her to take Arpita's place...Or may be, he is just scared that he will soon start liking Arti, which will be an insult to Arpita's memories...😃