Pietro was Leorio's close friend. He died from an illness that was actually curable—it just needed surgery that Leorio couldn't afford at the time. The loss hit hard. Leorio's been carrying guilt ever since, knowing he couldn't save someone he should have been able to help. That's what drives him now: the determination to become a doctor so he never ends up in that situation again. It's why he entered the 287th Hunter Exam in the first place.
Content compiled by AnimeList.moe from publicly available sources.
