No, motorcyclists do not always flash their headlights at other motorists when passing them on the road. Flashing headlights can be a form of communication between drivers, but it is not specific to motorcyclists.
Motorcyclists, like any other vehicle drivers, may flash their headlights for various reasons. It could be a way to alert other motorists of their presence, particularly if they feel they may not have been noticed or if they are trying to overtake another vehicle. Flashing headlights can also be used as a way to indicate a hazard on the road or to communicate with other drivers in certain situations.
Some motorcyclists may choose to flash their headlights as a courtesy or to indicate that it is safe for the other driver to merge or change lanes. Others may not use this practice at all. Ultimately, it depends on the individual motorcyclist's preference and the specific circumstances on the road.