Why does Nico Robin cry?

Nico Robin's Emotional Breakthrough

Overwhelmed with sorrow, she breaks into profound tears as she comes to the harsh realization that the world has stripped her of a reason to live. Until this moment, she had never been given the opportunity to voice her true desires. In a heartbreaking moment, she cries out her wish to live, asking Luffy to take her aboard the ship with the crew as they journey across the sea.