Most complaints are just requests that didn’t feel brave enough to ask directly.

Behind most grumbling is something somebody wanted and could not bring themselves to ask for directly. The complaint is safer because it does not risk a no, but it also cannot be answered, so it circles. Hearing it as a request rather than an attack changes what you can do with it.

When someone complains at you, try asking what they would like to happen. When you are the one complaining, translate it into a plain ask before it goes any further. A request can be declined, which is uncomfortable, and at least it moves.