Meaning: The doll was a reminder of the love and joy in the girl's life.
Choice of Words: "Symbol" refers to something that represents or stands for something else. "Love" is a deep affection and care for someone or something. "Joy" is a feeling of great happiness and delight. "Doll" is the toy figure. Combining these, "Doll became symbol of love and joy" implies the doll came to represent and embody the emotions of love and happiness in the girl's life.