The Bible teaches us various things, one of which is God's sovereign power and goodness. He remains in control of all things, and no matter what we feel or think, His plans are always better than ours.
Such is the truth we find in the life of the prophet Samuel.
A plan that failed
Samuel was a great servant of God. The Bible tells us that Samuel grew up with the LORD, and God did not allow any of his words to "fall to the ground" (see 1 Samuel 3:19). Simply put, this prophet accurately spoke God's Word, and proof of that is the actual fulfillment of what God says through him.
God was with Samuel during the time he ruled over Israel as judge. However, he had no one to replace him as he grew old (for his sons were corrupt – see 1 Samuel 8:3). The people of Israel then clamored for a king to lead over them. The Bible tells us that this king would be a man named Saul.
Saul was a choice man, someone who towered over others in Israel (see 1 Samuel 9:2). Average human thinking would say that someone with bearing would make for a fine leader. God knew better – He wanted Israel to look to Him, but since they were stubborn He gave them a king: He commanded Samuel to anoint Saul (see 1 Samuel 8:22).
Later on we read that Saul displeased the Lord several times, causing God to reject him and take away the throne from him (see 1 Samuel 13-15). But there is an important lesson we can learn from Samuel's mourning over Saul (see 1 Samuel 15:35).
1) Failures happen
From chapter eight to fifteen, we read of Samuel continuously warning Israel to choose God over man. He did his best to point people to the Lord, but the people were so stubborn. Despite Samuel's standing as "the" man of God during his time, people still wanted what they wanted.
Friends, failures happen. No matter what we do, we can't expect everything to go our way. One of the Bible's greatest prophets faced a failure in the form of a disgraced king that he himself anointed. Think about that.
2) Depression impairs our ability to serve the Lord
First Samuel 15:35 tells us that Samuel "constantly" mourned for Saul. Mourning is a normal, healthy response to dealing with loss or failure. Too much of it, however, cripples us and prevents us from doing what we are supposed to do for the Lord.
First Samuel 16:1 tells us that God doesn't want us to keep mourning for long. He told Samuel, "How long will you mourn for Saul, seeing I have rejected him from reigning over Israel?" God doesn't want us paralyzed by depression. He wants us moving on.
3) God's plans cannot be stopped by dead ends
At the time, Samuel must have thought that his ministry has ended. He was old and the man he anointed to take the reins after him failed. God, however, didn't see that as an ending; He saw that as an opportunity to do what He willed.
In 1 Samuel 16:1, God told Samuel to stop mourning, "fill [his] horn with oil, and go." Saul might have failed, but God still had an assignment for Samuel – to anoint the next king of Israel, the man named David (see 1 Samuel 16:8-13).
Friends, we might feel like God won't use us anymore because of our age, past failures, or even our public reputation, but that's not true. God's not done with us yet; while we're still alive He wants to use us.