She is under assumption that the prince is devoid of emotions until interacting with the chosen girl (heroine)
Which is true, but she misunderstood the underlying reason (and why her actions have thus been fruitless in shifting the prince's attention)
In truth, it's recorded that every now and then the royal family will birth someone extraordinarily gifted.
The thing is, these gifted people tend to be rather...'robotic' until one day something sparked their interest, then it become the only thing they care about from then on.
The first person was a king who just didn't care about anything except his childhood friend, who ends up being the one to kept him a good king.
Some from then on became interested in war, and became a warmonger.
Some research and became a shut-in researcher (albeit producing many results)
So by the MC's actions early on, she already piqued his interest and has imprinted herself as his 'chosen one' since long ago.
She wanted to save him from the fate of being a doll prince and thought it required the heroine to do so, thus her firm determination to be 'villainess'