Shulchan Menachem. To conclude Chapter Reish Lamed Beis, 232, the Rebbe explains the unique power of reciting the Korban Pesach on Erev Pesach.
Korban Pesach represents the time of our freedom—both physical and spiritual. Physically, we were freed from slavery in Mitzrayim. Spiritually, we were transformed from servants of Pharaoh into Avdei Hashem. This represents the highest level of true freedom.
Because of this, the recitation of Korban Pesach reaches an exceptional level. It is not only that one gains the results of a korban, nor only that it is considered as if one is performing the avodah. Rather, it reaches a level in the cheftza itself—almost as if one is actually bringing the korban.
This creates a powerful experience, as if one is standing in the time of the Beis HaMikdash and participating in the avodah in a very real way.
Yet precisely because one comes so close, it awakens a deep sense of loss. A person feels that although he is almost there, it is still not the actual korban. The true fulfillment can only be achieved through the physical bringing of the Korban Pesach in the Beis HaMikdash.
Therefore, the Alter Rebbe concludes that one should feel pain over the Churban and pray to Hashem to rebuild the Beis HaMikdash, so that we may once again bring the korban in its fullest and most complete form.
Summary
Korban Pesach represents the highest level of freedom and connection, reaching almost the reality of the korban itself. This closeness intensifies the longing for the Beis HaMikdash, leading to heartfelt prayer for its rebuilding.