From Volume 17 - Chapter 152/ Season 2 Episode 90. With Wapol defeated and Nami cured, the Straw Hat Pirates have everything they need to head to Alabasta. Despite this, Luffy refuses to leave without a new crew member. A doctor is just what he needs to continue his journey. Unfortunately, Chopper is refusing the offer.
(Note: Sorry I'm a day late. There were internet issues.)
From Volume 17 - Chapter 146 / Season 2 Episode 87. Being chased around by Luffy and Sanji, Chopper heard them calling him a monster, just like every other human. So imagine his confusion when they start chasing him again, begging him to join the crew. It's not until Wapol climbs to the castle and gets walloped by Luffy's rubber powers that Chopper finds out what the word "monster" means to the Straw Hat Pirates.
From Volume 42 - Chapter 407 / Season 5 Episode 290. Losing his fight against the CP9 agent Kumadori, Chopper is at his wits end. There is one Hail Mary he's capable of, but will he be able to keep his humanity?
