I don’t know the origin of this quote. It’s certainly not the Buddha, and the modern phrasing sounds more like Sharon Salzberg or Jack Kornfield. I even wondered if it might be something I’d written.
The Buddha certainly did encourage the development of kindness (metta) as the basic way or relating to others. In a conversation with Cunda, a silversmith, regarding how one purifies oneself, he said:
And how is one made pure in three ways by mental action? … He bears no ill will and is not corrupt in the resolves of his heart. [He thinks,] ‘May these beings be free from animosity, free from oppression, free from trouble, and may they look after themselves with ease!’
On another occasion he said:
Therefore, bhikkhus, you should train yourselves thus: ‘We will develop and cultivate the liberation of mind by lovingkindness, make it our vehicle, make it our basis, stabilize it, exercise ourselves in it, and fully perfect it.’ Thus should you train yourselves.
And in one of his most extensive discourses, the Karaniya Metta Sutta, he describes how kindness should be cultivated for all beings at all times:
Just as with her own life
A mother shields from hurt
Her own son, her only child,
Let all-embracing thoughts
For all beings be yours.
Cultivate an all-embracing mind of love
For all throughout the universe,
In all its height, depth and breadth —
Love that is untroubled
And beyond hatred or enmity.
As you stand, walk, sit or lie,
So long as you are awake,
Pursue this awareness with your might.
As a writer, I have to say that “Kindness should be the natural way of life, not the exception” strikes me as being a poorly constructed sentence. “Natural way of life” is being contrasted with “exception” which I don’t think really works, since these expressions are not of the same kind. “Kindness should be the rule rather than the exception” would work.
This quote is often found in the form “Kindness should become the natural way of life, not the exception.”