1. The painting Mona Lisa itself has extremely high artistic value and historical significance, which makes Luo Ji feel awe and shocked, thus triggering his inner feelings and mood swings.
2. Mona Lisa's smile and expression may remind Luo Ji of the person or thing he cherishes, and this kind of emotional singing touches his heart.
3. In Three-body, Luo Ji is portrayed as a rational, calm and decisive hero. This kind of crying may be a way to break through himself, showing his inner softness and sensibility, which may make him more real and three-dimensional.
4. Luo Ji may also think of his ex-girlfriend Zhuang Yan, which is a great sacrifice for him. Zhuang Yan's departure made Luo Ji deeply feel the pain of loss, and Mona Lisa's smile may have touched his deep sadness and thoughts.
The above analysis is for reference only. The novel Three-body is a science fiction novel created by Liu. The characters and plots in the novel are fictional and have no direct connection with the real world.