You shall not bear false witness against your neighbor. (Exodus 20:16)
We should fear and love God that we may not deceitfully belie, betray, slander, or defame our neighbor, but defend him, [think and] speak well of him, and put the best construction on everything. (Martin Luther)
Want a gauge of how serious God was about this commandment? Deuteronomy 19:18-19: “The judges shall investigate thoroughly, and if the witness is a false witness and he has accused his brother falsely, then you shall do to him just as he had intended to do to his brother. Thus you shall purge the evil from among you.” Let that sink in. If some one falsely accused another in court of theft, the false accuser paid the penalty for theft. If some one falsely accused another in court of murder or some other capital crime, the penalty would have been death. The penalty for perjury was proportional to the seriousness of the perjury. This provided several protections against perjury to Israel's society: it was a heavy deterrent against corrupting justice in its courts with perjury; it made vindication of the falsely accused very public; it enabled a put-up-or-shut-up challenge for victims of gossip or slander.
The positive duty Luther adds in his explanation is both meant to prevent perjury, slander and gossip and to improve society generally. Being kind and polite may sometimes seem irksome and phony, but it also makes neighborhoods and cities more pleasant places to dwell and do one's daily business. Personally, I find these positive duties quite challenging. Defending someone from gossip is not usually a way to popularity, especially with the gossiper (BTDTGTTS!). On the other hand, it takes small stuff like that, by many, frequently and over time, to encourage civility in society. And “put(ting) the best construction on everything,” not being suspicious, not assuming the worst about things I don't understand? Even with people I dislike? That is tough for me to do! Then there's the question of what I permit around me. Do I let others tickle my curiosity through gossip? Or do I remind gossipers that their gossip is none of my business and of doubtful veracity? Do I encourage this social poison by listening to it? Or do I do my very small part to make my community a slightly better place (remembering the effect of hundreds or thousands of others doing that same very small thing).