One can imagine the great feeling of liberation experienced by the Israelites when they were finally freed from slavery after so many years. We see from here that even though the irritation experienced would have been slight under the circumstances, it would have nevertheless still been a blot on their joy. From here we can learn that when someone is experiencing a joyous occasion, we should be careful not to say or do anything that would decrease his joy. A person might have just bought a new house and feels very happy about it. At that time don't needlessly point out the drawbacks of that house.
Some people have a tendency to make statements that deflate a person's high feelings.
They might be motivated by envy, or they could be simply insensitive. Allow others to savor their good fortune. Don't be like a barking dog and cause others irritation. When Hashem created the world, the two luminaries, the sun and the moon, were of equal strength. In response, Hashem made the moon smaller.