In the first John Wick, a gangster attacks John Wick in his home, not knowing who the man is, steals his car, and kills his puppy. However, the puppy was a final gift given to John by his now deceased wife, which makes the dog a vital connection for him. It leads him to seek revenge, but that act of seeking revenge then leads him down a path that results in the two sequels. Hundreds of people dead in every imaginable way, all because of the dog.
Article source: https://www.cinemablend.com/news/2480198/john-wick-chapter-3-director-chad-stahelski-admits-the-movies-are-wacky