She firmly believes that there is no eternal love in the world, so she should break up when she loves and stay together when she doesn't love.
She couldn't tolerate it. They fell in love and had to separate in the end, so in order to maintain their love, it was just right to break up at this moment.
Six years ago, they broke up when she loved him the most, and she went abroad; now she doesn't love him anymore, and they want to be together for the rest of their lives.