When ha was small, he ate a devil fruit. Because he was different afterwards, he was left out and beaten up by the other animals (his parents). A doctor found him heavily wounded. Doctor befriended him. Chopper got greatfull over time as the doctor didn’t try exclude Chopper. Chopper got his student and foster child. Chopper tried his best to also treat the doctor becquse he was about to die so he studied hard. He found a cure, a mushroom. He went out and found it. The mushroom was marked with a skull in the books, wich meant freedom for him as the doctor was a pirate fan. Doctor knew that the mushroom was poisned but he ate it anyway because he saw how happy Chopper was when he brought it, despite being hald dead. The Doc ate it. The Doc died. Chopper was once again alone. Chopper got taken in by an other Doc (the old woman) wich told him, that Chopper gave the Doc a poisned mushroom and killed him. Chopper was devasteted and wanted to learn more about medicine so the mistake didn’t happen again. His dream is to be able to heal every illnes.