Chopper is a kind person, despite the abuse we just saw, because he cares for the helpless. There's a reason Sanji is one of the people here to meet the young doctor. Remember his intro? Beat the shit out of a guy, then selflessly feed a starving man. Nami's introduction had her conscience keep her from murdering a tied-up Luffy with a Buggy Ball. Luffy stood up for the wimpy Coby. Zoro ate a dirt ball so a little girl would feel better about her cooking. Even Usopp lied to his little kid friends to get them out of danger. Chopper saves animals because Chopper is an animal. Chopper saves babies because Chopper is baby. But just like with Sanji, Luffy doesn't care so much how tough the new kid is.
Chopper is a kind caregiver and THAT is the requirement to be a Straw Hat. Not strength. Not clout. Hell, Usopp didn't have his title of marksman until after the crew left his island. Luffy just wants interesting and nice people to be his family.
