So this is basically Vansh's problem.
He is jealous. Purely jealous that he isnt her "first love"...that she didn't feel love for him when she met him (as she was in love with someone else)...which goes to imply he fell for her the day he saw her.
+1
Vansh is definitely jealous for not being Riddhima's first love! In fact he hates to accept the fact that Kabir used to be her so called " First love"
Remember When Kabir said "Main pehla pyaar hu tumhara" How Vansh screamed "Enough"
That scene said it all!😆