THE OU'S Taryag: The 613 Mitzvos mitzavah 15. No Doggie Bags: The prohibition against removing the korban Pesach from its designated group

As with all the mitzvos pertaining to the korban Pesach and to Passover night, the reason for this is to celebrate the miracles that God did for the Jews when He took them out of Egypt. In this instance, it is to emulate royalty.