When Mordechai alerted Queen Esther about Haman's decree, she told him: "Go and gather together all the Jews and fast for me for three days." Why did she ask him to gather the Jews together? Why didn't she just tell him that the people should fast?
Esther realized that there was a reason why Haman was given the power to destroy the Jews. This could not have happened by itself. The Jewish people must have been doing something wrong if such a terrible decree was threatening them.
Haman also knew this. He had seen the Jews' weakness and planned to use this opportunity to destroy them.
What was that weakness? What were the Jews doing wrong?
If we look closely at what Haman told King Achashverosh, we can understand. He said: "There is one nation which is dispersed, scattered, and separate among the people. They do not heed the king's laws. They should be wiped out."
When Haman said, "dispersed, scattered and separate," he did not mean only that the Jews lived in different places throughout the empire. He meant that they were separate from each other. Instead of caring for others and showing unity, each person was concerned with his own affairs.
Haman knew that when the Jewish people are united, they are strong. Since they were "scattered and separate," he figured that this was the right time to try to destroy them.
But the Jewish people really are a nation of achdus and ahavas Yisrael. We all have a neshamah that is part of HaShem and that unites us.
This is what Esther was thinking. She knew that it was necessary to bring the Jews together again and that is what she told Mordechai to do.
Esther was right. The Jews gathered in prayer and revealed the ahavas Yisrael which makes us a strong nation. Now Haman's plan could no longer work. He suffered a well-deserved ending and the Jews celebrated this great miracle by establishing the holiday of Purim.
We celebrate Purim in the spirit of ahavas Yisrael. We send mishloach manos to our friends and give matanos l'evyonim to the poor. Just as in the time of Purim, by gathering together and performing these mitzvos, HaShem will give us "light, happiness , gladness, and honor."
(Adapted from Sichos Purim 5722; Sichos Purim 5727)