In response to this, Shammai taught his lessons. Certainly in worldly and interpersonal matters, one should be flexible and accommodating. But this is not always appropriate behavior. In spiritual matters, one must be strict. This distinction is based on a fundamental difference between the physical and spiritual dimensions of man.
Physical matter is in a state of constant change.
Spiritual reality doesn't change. If we don't grow spiritually, we regress. This is because we are physical beings with a spiritual component. These purely spiritual beings have no element of progress or regression. Their reality and level is fixed.