Our Lord always speaks the truth, but when he delivers truth with this particular introduction it is to call the minds of his hearers to special attention. Sheepfolds were walled areas where flocks were gathered for a time, for instance, overnight, so that the shepherds could sleep. But he that entereth in by the door is the shepherd of the sheep. He who is authorized to be caretaker of the sheep need not attempt entry by some other means. Though there may be many flocks resting within the sheepfold, each flock recognizes its own shepherd and responds to his call.
Sheep naturally follow the shepherd from pasture to pasture.
The shepherd is responsible for their care and safety. That which is familiar is comfortable and feels safe. Probably the Pharisees, leaders of the Jews, who were present when Jesus revealed himself as the Son of God to the man born blind, near the end of the previous chapter. Verily, verily, I say unto you, I am the door of the sheep. It is through me alone that mankind can be saved.