Because she's trying to push him away
They are hurting themselves at this lmao, Shes told them both repeatedly that shes not looking for a relationship and they've both ignored that and decided to cling on. The problem is they are both hoping that if they continue having a sexual relationship with her that she'll magically fall in love them which is wrong cuz doona is under the assumption that they're on the same page and that's not case.
point*
That was her goal. They are both wanting a romantic relationship with her when all she wants is a sexual relationship. When she's ready she'll be more accepting but right now she's not and Gyrin and Haesol need to respect that. I don't know what else Doona could do, she's rejected him numerous times and abandoned Haesol by moving away but he still pursued her. At this point we can't continue to blame Doona if Haesol gets hurt, he chose this. The same can be said for Gyerin.
Doona has some things she needs to work out before she's ready to commit to another relationship. She's not ready right now, that's all. I think the best thing Heasol could do is stick with fwb with her. Surely she'll open up to him more with time. Pressuring her to be in a romantic relationship will only cause her to push him away more. So he needs to decide if he wants to accept the relationship as is or move on.
Why would she hurt him like this?
My boy you've fallen in love with the wrong person... -_-