Although Ruve (as said above) swears he won't force her. Ruve managed to win an extra year before they decide between the two girls and hope to win Tia's heart by then. But (as explained above) she wears Carsein's dress and refuses to consider going to him.

In fact Ruve's love for Tia takes roots very early in the novel.

As we saw, Ruve already shows signs of caring for Tia when she's visiting the emperor while being sick, and he takes her back himself in his carriage. Then on the way back he thinks fondly of her, because he sees the similarities between him and her, and secretely wishes for her to get better quickly.

Later when when they dance on the ceremony of his coming-of-age, we can see that Ruve is displeased that Tia seems close to Allendis while she's cold to him - unlike in the 1st timeline, Ruve doesn't let Tia fall and instead presses her close to him, masking her misstep, and Ruve explicitely shows jealousy towards Allendis because Tia shows her emotions when it comes to Allendis - he says she probably would have preferred to dance with him.

Ruve concludes by saying "how unpleasant" (that you show your emotions to another man, that you're good at dancing so you probably practiced with another man, that you are wishing you were dancing with another man, that you're intimate with another man while you're cold and expressionless with me, etc).

We know for sure that Ruve's attitude towards Tia has changed when he pays a visit to her on her estate and finds her with Carsein. Ruve calls Tia HIS FIANCEE as much as he can and straightforwardly tells Carsein to stay away from Tia, while at the same time confronting Tia on her rejection of their engagement.

He obviously doesn't like to inspire fear to her.

And when he reads the letter Allendis wrote to Tia, he's outraged that Tia thinks he could marry another woman. The idea never even crossed his mind (and he will be true to his word : Tia will be his only love and his only wife). So we can see Ruve's love takes its roots really early in fact. But he has a very introverted personality and he's very cold and composed in public, as a royal should be.

Ruve falls in love with Tia also because of how expressive and lively she is in the second timeline, and also because, if you've forgotten, he loved Jeremiah (Tia's mother) because only Jeremiah and Zena ever showed him love and affection, and Tia resembles Jeremiah A LOT.