In John 21:17, Jesus uses the question, "Do you love Me?" to remind us of the great commandment to love God with everything that is in us. He uses the imperative; "Feed My sheep!" to focus our attention on the second great commandment to love our neighbors as ourselves. As you know, Jesus said that these two commandments summarized the Law and the Prophets. In the context of the Gospels, the risen Savior's dialogue with Peter (before the other disciples) in this passage is an appropriate closure to His ministry in the Gospel of John. It is, likewise, a call to every believer to ministry, to the specific ministry of loving and worshiping Jesus and of ministering to His sheep.
The correlation between the question (Do you love Me?) and the imperative (Feed My sheep!) is clear. To have a heart for God is also to have a heart for that which concerns Him. And God has made it known throughout the Scriptures that He is passionately concerned for His people. The Great Shepherd tends to His sheep. He watches them; He feeds them, and He protects them. He makes them to lie down in green pastures and leads them beside still water. He cares more deeply than we can know. Today, He most often uses fresh and blood, in the form of you and I to accomplish these important tasks.
If we are to be like Him, we too, must have the heart of a shepherd which will lead us to passionately care for the welfare of others. It has been observed that, too often, we envision our love for God as a separate endeavor from our love for others; but the two are intertwined -- the first, if genuine, invariably leads to the second. John spells this out in his first epistle: "to love God is to love our brother." If we are to have any genuine relationship with God at all, we must feed His sheep.