Why do Japanese people always bow?

According to the book Being Upright: Zen Meditation and the Bodhisattva Precepts
By Tenshin Reb Anderson.

In Soto Zen one of the ways of receiving dharma from our teacher is receiving our teacher’s manner of bowing. When I join my palms together in a gesture of respect toward any being, animate or inanimate, I feel the hands of my teachers embodied in my hands. I feel my teachers’ presence in my joined palms. I also feel the presence of their teachers, all the way back to Shakyamuni Buddha.

Dogen said that when there is bowing in this world, the buddha way flourishes; when there is no bowing in this world, the buddha way perishes.

Many Western students of Zen have some resistance to the formal practice of bowing, especially the practice of full prostration. This resistance can manifest itself as being unwilling to practice the bow or being overly enthusiastic in practicing the bow. Such resistance is good: it is through working with it that you find the balanced, upright attitude with which bowing is practiced by the buddhas. Finding the place of balance between belligerence and obsequiousness reveals the selfless path of the awakened ones, where a bow is just a bow.

The practice of bowing offers an opportunity for cutting through all dualities of self and other, sentient being and buddha.

As Suzuki Roshi said, “When you bow, there is no buddha and no you. One complete bow takes place. That is all. This is nirvana.