@Jihan Putri maharani doc Dan leaves jaekyung's house with a letter and he also quits his job and went away with his grandma his grandma wanted to visit a place that she went to when she was a kid .doc Dan said in his letter that he would pay all the money back to jaekyung and jaekyung is angry after finding it out
Dan resigns from working at the gym and as Jaekyung's physical therapist because he feels like he's responsible for the outcome of the last match (which Jaekyung now knows is not the case). His grandmother is also dying and wants to see the ocean one last time before she passes, so Dan has checked her out of the hospital and is taking her to the ocean. He took information on Hospice care with him, as there is nothing what medicine can do for her, so he just needs to make her comfortable. Thus, Dan has left Jaekyung's house and traveled to the ocean with his granny.
Jaekyung is actually freaking out that Dan left (too little, too late buddy), and is feeling like sh*t. He deserves every ounce of what he's getting right now. So I believe Mingwa is setting the second season up for them being apart for some time, and Jaekyung realizing he misses/loves Dan, chases after him, etc.