Why is Ethan uncomfortable when he runs into Sophie in Maui in the novel, The Unhoneymooners?

Sophie is Ethan's ex-girlfriend who Olive and Ethan run into while on their fake honeymoon in Maui. Sophie is on the island celebrating her recent engagement to her fiancé, Billy. It is revealed that Sophie and Ethan were together to the point where Ethan proposed to her. She never actually said yes and ended up moving out their apartment that night. Ethan was devastated but later realized that their relationship lived on the surface and what they had wasn't real. He had changed himself and his personality to be the man that Sophie wanted, even though it wasn't who he actually was. When Sophie runs into Ethan in Maui, she is evidently jealous of his marriage to Olive and tries to rekindle things. Ethan shuts down her advances as he realizes he no longer has feelings for her but instead cares more about Olive.