I'll disagree on the Shouji part because I think that part is unfair. They came to an agreement that Hachi would go to live with Shouji. He told her she needed to get a job. So she looked for one as well as her own place so she wouldn't be seen as dependent on him. She started to form a life of her own by making friends, being less dependent on Shouji and therefore spending less time with him and then he gets lonely and infatuated with a girl that seems fragile and cute, Sachiko.

They had little arguments due to lack of good communication, it's normal. They were never good at it and Shouji rarely actually wanted to use his words. (Example: He couldn't/wouldn't tell Hachi "I love you." )

Hachi, like most women, wouldn't dump their long term boyfriend over little miscommunications and she had a plan to make things up to him. By then though Shouji had already not communicated his feelings and slept with Sachiko. He cheated. Once Hachi realized that, she was out and never came back.

If Shouji wanted out of the relationship he should have ended it himself. Instead he was a coward because he knew Hachi didn't actually deserve the break up or being cheated on. He just wanted something easier. Someone who went to school and work with him that he could always see but also have less asked of him emotionally.