Diary of a Madman(1968)

Original title: Записки сумасшедшего

In the main character's diary, he describes his life, his work, and the people around him. He goes on to write about his feelings for a woman, and soon after that he begins to show signs of insanity - he talks to her doggie Meji, he gets hold of letters that Meji wrote to another doggie. After a few days, he's already completely disconnected from reality...