Why do we mourn during Sefirat HaOmer if it’s meant to be a time of excitement for Kabbalat HaTorah?

The period of time between Pesach and Shavuot  should be a period of aspiration and elevation   for the excitement of the Jewish people having  been liberated from Mitzrayim and ultimately   coming to the receiving of the Torah. However,  this period of time over the course of our   history has also been a period of time where we  have experienced tremendous Jewish tragedy. The   loss of the 24,000 students of Rabbi Akiva during  this period of time is a tragedy for the Jewish   people because these were great Torah scholars who  ultimately lost their lives because they did not   know how to create connections of love and respect  between each other. In order to receive the Torah,   we have to be a unified people. And so therefore  even a period of time as exciting as exciting as   Sefirat Ha'Omer is ultimately influenced by this  sentiment of a diminished sense of joy. Sefirat   Ha'Omer does become a period of mourning, mourning  the loss of the unity of the Jewish people and   appreciating the value of that unity as we inch  our way towards the world of Kabbalat HaTorah.