In Isla and the Happily Ever After by Stephanie Perkins, a character remains with a woman for an extended period because he yearns for the feeling of being in love. His actions are driven by a deep desire to experience love's emotional fulfillment, even if it is temporary or based on an illusion.
He feels an emptiness within himself, represented as an 'empty well' that needs filling with the love and connection from someone else. This longing highlights his internal struggle and need for emotional closeness, which he seeks through relationships, even if they may not always be fully satisfying or genuine.