When I speak, my words are master over me. When I do not speak, I am master in that I withhold them (Orchos Tzaddikim, Chapter 21).
Everyone has an inherent drive for power and control. We may use it for evil; for example, we may seek control over other people. On the other hand, we may use it for good and try to control our own drives and urges. In any case, it is often frustrating to discover that something is beyond our control.
Words are within our control until we have spoken them;then, we cannot control their effects. At the very best, we can retract what we have said, but that only sets up an opposing force to that which we have created. The original words can never be recalled. We often find ourselves powerless and subjected to the consequences of what we have said, in which case the words we have spoken have indeed become our masters.
How do we avoid this feeling of powerlessness? We have to take control of our speech and learn to keep silent when we have nothing constructive to say. If we do have to speak, we should choose our words very carefully.
If we had to choose a boss, we would certainly be very careful in our selection. We should be no less cautious with words.