"Surprisingly, the shepherd discourse does not begin with the words: 'I am the Good Shepherd' (Jn 10:11), but with another image: 'Truly, truly, I say to you, I am the door of the sheep' (Jn 10:7). Jesus has already said: 'Truly, truly, I say to you, he who does not enter the sheep-fold by the door but climbs in by another way, that man is a thief and a robber; but he who enters by the door is the shepherd of the sheep' (Jn 10:1f). This can only really mean that Jesus is establishing the criterion for those who will shepherd his flock after his ascension to the Father. The proof of a true shepherd is that he enters through Jesus as the door. For in this way it is ultimately Jesus who is the Shepherd -the flock 'belongs' to him alone.
In practice, the way to enter through Jesus as the door becomes apparent in the appendix to the Gospel in chapter 21 -when Peter is entrusted with Jesus' own office as Shepherd. Three times the Lord says to Peter: 'Feed my lambs' (or sheep -cf. Jn 21:15-17). Peter is very clearly being appointed as the shepherd of Jesus' sheep and established in Jesus' office as shepherd. For this to be possible, however, Peter has to enter through the 'door.' Jesus speaks of this entry -or, better, this being allowed to enter through the door (cf. Jn 10:3)- when he asks Peter three times: Simon, son of John, do you love me? Notice first the utterly personal aspect of this calling: Simon is called by name -both by his own personal name, Simon, and by a name referring to his ancestry. And he is asked about the love that makes him one with Jesus. This is how he comes to the sheep 'through Jesus': He takes them not as his own -Simon Peter's- but as Jesus' 'flock.' It is because he comes through the 'door,' Jesus, it is because he comes to them united with Jesus in love, that the sheep listen to his voice, the voice of Jesus himself -they are following not Simon, but Jesus, from whom and through whom Simon comes to them, so that when he leads them it is Jesus himself who leads.
The whole investiture scene closes with Jesus saying to Peter, 'Follow me' (Jn 21:19). It recalls the scene after Peter's first confession, where Peter tries to dissuade the Lord from the way of the Cross, and the Lord says to him, 'Get behind me,' and then goes on to invite everyone to take up his cross and 'follow him' (cf. Mk 8:33ff). Even the disciple who now goes ahead of the others as shepherd must 'follow' Jesus. And as the Lord declares to Peter after conferring upon him the office of shepherd, this includes accepting the cross, being prepared to give his life. This is what it means in practice when Jesus says: 'I am the door.' This is how Jesus himself remains the shepherd."
-Pope Benedict XVI in Jesus of Nazareth.