David's succession to Saul features a number of unusual characteristics.
For one, in the ancient world kings were usually succeeded by one of their sons, or by a brother if there were no sons. Sometimes non-family members took the throne on the death of a king, but that was usually because they had led a successful revolt and had caused the death of the king. On the rare occasions when a non-family member succeeded peacefully, the king himself usually chose who he wanted.
He would, of course, try to choose the best person for the job. The ablest military commander, the best legal mind, the wisest diplomat -- in fact, a king would probably try to look for the same qualities if he intended to choose from among his own sons. The primary goal was to select someone who was ready to be king and so minimize the amount of palace intrigue and maybe even bloodshed that could result from a poor choice.
So absolutely none of those usual procedures were followed in the selection of David. He's not related to Saul, Saul didn't pick him, and although he's shown some courage by facing down animals that threatened his sheep, he's too young and inexperienced to have made a good king.
And even more interesting, once he's anointed absolutely nothing changes for him. He's still the one who watches the flocks and he's the one too young to fight who'll be sent off to get word from his brothers when they're part of the war with the Philistines. Although in the eyes of Samuel and probably his own immediate family he's now the next king over Israel, his life proceeds as it did before. In fact, read the story of his visit to his brothers and you'll find that, king-to-be or not, they still see him as their troublemaking little brother.
But David's...interesting selection can tell us some important things about how God works, which might help us understand something about our own calls to serve him.
First, as we see in the story David was not the obvious choice to be king, even for Samuel who had spent a lifetime as God's servant. Samuel sees the tall and strong older sons of Jesse and believes they look like kings, so God must have chosen one of them. But God tells Samuel that his criteria for kingliness don't necessarily match human criteria. After all, Saul looked the part of a king as well, but proved to be spiritually too weak to lead God's people.
Next, we see that God is willing to wait until the person he selects is present. Had Jesse's sons all been present, he could have led them before Samuel youngest first just as easily as he did this way. He could have had them in a group and let Samuel see them. The arrangements of the meeting ensure that the choice God really wants to make won't be shown to him until the very end, but he will wait and not settle for less than what or who he wants.
And we see that God's choice may not bear full fruit right away. David is just a boy now and won't be king for many years, but God makes his choice now in spite of all the time that will be involved.
You and I are called by God for roles just as surely as was David. We are probably not called to be the kings of Israel -- because the throne room would get very crowded -- but we are each called. Just like David, we are each called for a purpose God intends for us to fulfill. Oftn we tend to hang back from the idea God calls us to something, saying we're not up to it or that God must surely have someone around who could do better than we could.
Really? Do we really expect God to say, "You're right! I don't know how I could have messed up like that! Good thing you knew yourself better than I did so you could set me straight!" We don't, but backing away from God's call out of a false modesty amounts to the same thing. Not a one of us is worthy of that call, but God has made it and promises us we can fulfill it with his help.
We may have been called long ago for a role that has yet to come to pass, just like Davdi waited long before he was king. But just as surely as he was anointed for his role long before he stepped into it, those who still wait to learn what God wants of them have been anointed for whatever that is.
Plus, there may be a thing or two we can do for God in the meantime. You may remember David got one or two little matters out of the way before he became king, too.