Though every action may be said to be "Zazen" in its limitless meaning ... whether walking, standing, running, sitting or reclining ...
One reason is that it can be a position of ease and balance ...
Question Six:
Q. [Someone
asks], Among the four different postures (walking, standing, sitting,
lying down), why does Buddhism encourage entering realization through
meditation only in sitting?
A. It
is not possible to thoroughly comprehend the path by which the buddhas
from the past, one after another, have been practicing and entering
realization. If you seek a reason, you must know that it is only
because [sitting] is what has been used by Buddhist practitioners, and
beyond this you do not need to search. However, the Ancestor
[Nagarjuna] praises it, saying "Zazen is the dharma gate of ease and
delight (nirvana)." So, couldn't we infer that it is because [sitting]
is the most stable and peaceful of the four postures? Moreover, this is
not the path of practice of one or two buddhas, but all buddhas and
ancestors follow this path.
From: Talk on the Wholehearted Practice of the Way - Kosho Uchiyama (with Shohaku Okumura, Taigen Daniel Leighton)
