This is very popular council in the "self-help / feel good" circles, but it is also very problematic and contrary to Zen practice.
Suppose instead of "assume the best" the author gave the advice "assume the worst." Would the reader celebrate his advice? If the cycle of assuming the worst in the face of every question of motive generates a negative personality, we would shun this advice, would we not? We would not want to set up a cycle of behavior that reinforces negative emotions, would we?
But the Zen practitioner sees that "assume the best" and "assume the worst" are equally delusory.
When a common objective of Zen practice is to see clearly, why would we accept advice to ignore what is in front of us in favor of what we will create in our own thoughts? And why would we not scrutinize advice that deepens attached, pleasure-seeking behavior?
Sometimes we are in situations where we feel bad. Sometimes we are in situations where we feel good. This is not a problem. But, here you are in your current situation. Are you in control of yourself in spite of your circumstances, or are your circumstances in control of you?
So, how should you respond when the question of motive arises? The first koan in our Zen tradition considers a similar question:
A student asks Master Yun-Men, "Not even a thought has arisen; is there still a sin or not?"
Without hesitation, the master responded, "Mount Sumeru!"
Why did the master answer this way?
Can you catch the sense of this koan? What are your thoughts?