This is a discipline because our first instinct when evaluating the behavior of others is often to suspect ulterior motives. In most cases we are working from ignorance since we cannot see into the mind of others.
The problem with assuming the worst about the actions of others is not only that we are almost wrong, but that it causes us to respond in ways that are not helpful to a good relationship. After all, if I suspect that an individual has poor motives, I will respond with suspicion, distance and possibly even anger. My distancing from that individual in turn causes others to distance themselves as they watch our attitude. We influence others when we assume the worst rather than the best. Sometimes it is not possible to undo the impressions we share with others.
Buckingham has it right: Find the most generous explanation for each other's behavior and believe it. Doing so allows us to operate from a position of trust and regard rather than mistrust. We don't have to like all the behaviors of others but the reality is that they probably don't like all of ours either. But we always hope that they will assume the best because we intend the best.
In evaluating the actions of others it is worth considering that we can quickly jump to negative conclusions about them that we would never want them to have toward us. It is a superior attitude rather than one of humility, recognizing our shared humanity.
Jesus had it right when he told us to "Do to others as you would have them do to you (Luke 6:31)." That includes our assumptions about others and our generosity toward their motives. Jesus said in the same conversation, "Be merciful, just as your Father is merciful (Luke 6:36)." Both are keys to good relationships and to a less judgmental attitude toward others.