Ki Tisa: Memorizing Torah Shebichtav & Writing Torah SheBa'al Peh

The prohibition of writing down Torah Shebaal Peh and memorizing Torah Shebichtav. What is the reason for these prohibitions?

This shiur is reposted with permission from the VBM—The Israel Koschitzky Virtual Beit Midrash of Yeshivat Har Etzion