There are many species pokemon which injure themselves in battle, some of which have even gained fame for this tendency. Staraptor are known far and wide for their reckless courage, while Electrode have a reputation for blowing up when losing a battle. And then there are Pichu, which impresses even Wailord with their willingness to absorb and even be knocked out by their own attacks; if only they had the volts to back it up!
Pikachu and Raichu are known to forage over some distance for food, and they are poor diggers, so Pichu are often left alone at a young age. It is believed that their recoil attacks, such as volt tackle, evolved as a defense mechanism. When faced with hungry Pidgeotto or Fearow while their parents were out, Pichu had to rely on their own attacks for survival, and if they injured themselves in the process it was better than being eaten.
Sadly in this case, man has again perfected what nature invented. Today, there is a popular tournament circuit for newborn pokemon known as the Little Cup, and Pichu, with their powerful yet self-destructive attacks, have become a staple. Breeders strive to create stronger and stronger attackers at birth, caring little for how badly the baby pokemon injure themselves, at times even equipping dangerous items such as Choice Band or Life Orb to make their attacks stronger. Many Pichu will have shortened lifespans and permanent injuries even once they evolve, but neither the breeders nor the Pichu seem to care. Instead, they strive for victory in the Little Cup, and with it, the coveted qualification (and inevitable last-place finish) in a more prestigious tournament which even Mewtwo is said to never miss.