William MacDonald emphasizes the necessity of moving on from mourning over the past, as illustrated by God's command to Samuel to stop grieving for Saul, whom He had rejected as king. Samuel's prolonged sorrow hindered his ability to fulfill God's plan, which included anointing David as Saul's successor. MacDonald draws parallels to our own lives, where we may mourn lost opportunities, relationships, or dreams, but stresses that while mourning is natural, it should not paralyze us from embracing the present and future. He encourages a timely recovery from grief, reminding us that God has a purpose beyond our disappointments.