Q. Why do most people regard the Omer period as a time of sadness?
The classical tragedy was during the Bar Kochba rebellion in about 135-138 CE, when an epidemic decimated the students of Rabbi Akiva, who were part of the Bar Kochba campaign. The generally accepted statistic is that 24,000 students died, though there are other versions of the numbers involved.
Other events at this time of year range from the attack of Amalek in the wilderness to the Chmielnicki massacres in Eastern Europe in 1648-49. No wonder that we associate this period with tragedy.
Yet there is also good reason to find reasons for gratitude at this season, ranging from the redemption from Egypt at the beginning of the Omer period to the Giving of the Torah as the seven weeks of the Omer reach their culmination.