XP

John Wick is a great movie. The following can be considered as minor spoilers and I'd recommend just watching the movie.

John Wick is a movie about a guy that goes around killing people for a whole movie. That by itself would make a huge amount of people dislike the movie. Thus they gave John Wick a really reasonable motive and turned the movie into a revenge plot.

Now the movie's about: Idiot kills John Wick's dog, John Wick seeks vengeance and kills a lot of Russians. That would make you somewhat sympathize with John, but it wouldn't really be enough to justify killing that many people.

Thus the the dog is introduced as a gift from his deceased wife and John Wick himself is a retired godly assassin. The dog was just a puppy and John only had it for a few days before it was killed. The reason John retired, was so that he could finally settle down with his wife. To retire he completed a suicidal, impossible task... and then his wife died a while later due to an illness.

Now it suddenly became very reasonable for John to go kill all the Russian gangsters because the puppy killer was the son of the head of the Russian crime syndicate and is being protected.