He mixed too much of his own character with the loving boyfriend he pretended to be. If he acted like a kind sweet considerate boyfriend then Kunj wouldn't have an issue with him and would trust him more to be with Twinkle so he wouldn't try to protect Twinkle because she didn't need protecting because Kunj would've thought Yuvi was a good guy. Kunj isn't the main reason why Yuvraj's plan will be defective, it's Yuvraj himself.