The people who were Tamei and were not able to bring a Korban Pesach came to Moshe to complain that they also wanted to have a chance to do the mitzva of Korban Pesach. Through this brave act they were Zocheh to have the Mitzva of Pesach Sheni in the Torah through them. What is so special about Pesach Sheini that instead of Hashem telling Moshe the Halachos Hashem waited for the people to demand it?
Rav Gedalia Schorr in Ohr Gedalyahu explains that Pesach is a great gift from Hashem. Normally for us to get something from Hashem we must make the first move towards Hashem and then he reciprocates by opening the floodgates. “Pischu Li… KiChudoi Shel Machat VaAni Eftach Lachem Pesach Shel Ulam”. You open up a miniscule opening for Hashem and Hashem will open a gigantic opening for you. The seforim tell us that on Pesach night Hashem skipped over the “Pesach” of Bnei Yisroel. Literally this means He skipped over the door of their homes by Makas Bechoros. But it is explained B’Derech Drush that that Hashem skips over the “Pesach” the needle sized opening that we are supposed to make for him. We didn’t make the slightest move towards Hashem in Mitzrayim yet Hashem ignored that and came to our rescue anyway.
Sefira is a time where after having received Hashem’s great chesed on Pesach we go back slowly and earn it day by day. This says the Ohr Gedalyahu is the Yom Tov of Pesach Sheini. It is the Pesach that we demanded. After working on ourselves over sefira we make our move to warrant the great gift of Pesach. That is why it came about through the demands of Bnei Yisroel. When we demanded Pesach Sheini Hashem opened up the Heavens and graced us with this wonderful opportunity. The whole point of this second Pesach was that the inspiration come from us below.
The Ohr Gedaliyahu follows through with this thought and says that the Mazal of the month of Sivan is “Teomim”, twins. The reason is that on Pesach, Hashem made the first move and on Pesach Sheini and during Sefira we made our move. On Shavuos these two efforts meet and are united on Har Sinai where Hashem calls us Yonosi Tamasi. Hashem kviyochol becomes "Tam" or whole, when we join him. We are not simply recipients of undeserving goodness from Hashem. We are inseparable twins with Hashem, but only when we make our gesture towards Him.