Well probably becuz some people love thrill in their life, my friend is an example of that she dump a green flag cuz he's to green flag and she wants a thrill in their relationship. Most of the times these kind of people are the red flags.
Because the sparks were never there with Kyle as it is with Matthias. I mean even as she denied her feelings for Matthias later she was just not attached to Kyle like she is with Matthias, in fact she admitted this close to the ending of the novel. Matthias is just the one who makes her heart flutter. Tbh there are just guys we as women just can't see in a romantic like no matter how close we are to them.
How can she love and be with Mattias and not Kyle?!! (╯°Д °)╯╧╧ I wish the Author make an unexpected ending and she will pick Kyle instead he's so green flag (=・ω・=)