I want to outline for you five points that I see in the verses of John 10:1-18. The first thing we should note is that this conversation in chapter 10 is related to what happened before in chapter 9. Jesus heals a man born blind and Jesus refers to spiritual blindness. The spiritual blindness and thief and robbers are the Pharisees from chapter 9. In contrast, Jesus refers to himself as the true gate and Shepherd.
First, Jesus has a legitimate right to enter by the gate unlike the thief and robber. The doorkeeper is a porter who has the keys. The porter opens up the sheep gate in order for the shepherd to lead the sheep out to pasture. The thief wouldn't have access to the doorkeeper and would have to break in through illegitimate means. The thief has the intention of using the sheep for his or her own needs. The Bible makes it clear that there are many false shepherds today who take advantage Jer 23:2; Jer 50:6; Eze 34:2-3.
Second, the sheep know the shepherd's voice. The sheep will run from a stranger, but there is an intimate understanding and knowledge that the sheep have of the shepherd. Jesus is referred in many places in the Old Testament as the shepherd. Isaiah 40:11; Ezek 34:23; Zech 13:7; Heb 13:20; 1 Peter 5:4. The voice that is referred to here means a sound or voice that the sheep identified with their owner. Jesus said he also knows his sheep by name. By name implies intimate knowledge or to know the essence of a person. God knows your name and knows you better than you even know yourself.
Third, Jesus describes himself as the gate which leads to salvation (green pasture) full and abundant life John 10:7,9,10. He isn't one of many gates, but only one. The benefit of going through this gate is that it leads to safety, prosperity and an abundant life. Jesus told his followers in Matthew 7:13-14 "Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. But small is the gate and narrow the road that leads to life, and only a few find it."
Fourth, the good shepherd lays down his own life for the sheep. He does this because he love the sheep. His does this as a part of his own free choice. In contrast, the thief and robber runs when trouble comes because they don't care for the sheep.
Fifth, the thief or robber is someone who enters by some other way than the gate. In Acts 20:29-30 Paul tells the church that he knows that soon after he leaves that savage wolves will come and some would arise even out of the church and distort the truth. This happened in both Galatia and Colossae and were the reason that Paul wrote those letters. He refers to false brothers in Galatians 2:4 who infilitrated their ranks to spy on their freedom and to seek to make them slaves again. The sheep should be aware that these false breathren still exist today and take advantage of the sheep if they can.
See also J.Vernon McGee's sermon about this passage
https://www.blueletterbible.org/audio_video/popPlayer.cfm?id=7715&rel=mcgee_j_vernon/Jhn
Commentary on this passage
David Guzik https://www.blueletterbible.org/Comm/guzik_david/StudyGuide_Jhn/Jhn_10.cfm?a=1007001
Matthew Henry https://www.blueletterbible.org/Comm/mhc/Jhn/Jhn_010.cfm?a=1007001
Chuck Missler https://www.blueletterbible.org/audio_video/missler_chuck/Jhn/John_Vintage.cfm#John_10_I_Am_the_Door
