Many things, including having manners, are held in high regard in Japan. If you do not have them, you run the risk of offending someone. Be it, not intentionally, of course. Have you ever stopped to think about all the small things you do in your home country that might offend someone elsewhere?
Even the smallest of things can matter in different cultures, like this Japanese custom that dictates that one should respectfully receive a meal from a host or friend by saying “Itadakimasu,” which roughly translates to “I humbly receive.” This is a cultural and not religious act – which everyone expects you to do.