There are two movies that come to mind when reading your question: 'A Man Called Otto' and 'Hachiko'. Both of them truly made me cry, and it was quite intense. However, I recommend starting with 'A Man Called Otto.' You'll be surprised at the beginning of the story, and you won't anticipate how the movie will end sadly. As far as I can remember, I cried throughout the three acts I divided the movie into (and in each act, I cried like crazy). I even had to redo my nighttime skincare routine because I cried so much :(