Why does Zoro leave the crew?

Why does Zoro leave the crew? Nami tries to calm him down, but Zoro angrily slams his sword on the ground, asking if leaving the crew is so easy to do. Zoro then goes on to say that he cannot trust a guy that decides to quit the crew then just come back.

Nami tries to calm him down, but Zoro angrily slams his sword on the ground, asking if leaving the crew is so easy to do. Zoro then goes on to say that he cannot trust a guy that decides to quit the crew then just come back.