(Review) Ep13 Lovely Runner

His watch lit up because allegedly the watch's time travelling abilities activates whenever Sun Jae dies, as per Im Sol's observations. So because she had already used up the one that Im Sol had and was using for time travelling, she can't use it to go back further. The one that Sun Jae had however, the one that he was wearing, activated, therefore she now has 3 new time travel chances. She used the one Sun Jae was wearing and went back in time to when Sun Jae fell in love with her at first sight to completely erase herself from his life. Because he always ends up saving her from the murderer if he knows Im Sol. So she erased herself from his life. Everything that happened in all the past episodes is now only in her memories. To Sun Jae, she is a complete stranger now.