It mentions a narrow door, but with no comparison. As opposed to what?
Luke 13:24 (KJV):
(24) Strive to enter in at the strait gate: for many, I say unto you, will seek to enter in, and shall not be able.
Strive , or struggle to enter the narrow or strait gate/door, i.e. it is a difficult to get through gate/door, but we should make the effort to do so, before the gate/door is permanently closed. As Jesus said in Matthew 7:13-14 (WEB):
(13) “Enter in by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and many are those who enter in by it.
(14) How narrow is the gate, and restricted is the way that leads to life! Few are those who find it.
or ESV:
(13) “Enter by the narrow gate. For the gate is wide and the way is easy that leads to destruction, and those who enter by it are many.
(14) For the gate is narrow and the way is hard that leads to life, and those who find it are few.
See also John 10:1-15 (WEB):
(1) “Most certainly, I tell you, one who doesn’t enter by the door into the sheep fold, but climbs up some other way, the same is a thief and a robber.
(2) But one who enters in by the door is the shepherd of the sheep.
(3) The gatekeeper opens the gate for him, and the sheep listen to his voice. He calls his own sheep by name, and leads them out.
(4) Whenever he brings out his own sheep, he goes before them, and the sheep follow him, for they know his voice.
(5) They will by no means follow a stranger, but will flee from him; for they don’t know the voice of strangers.”
(6) Jesus spoke this parable to them, but they didn’t understand what he was telling them.
(7) Jesus therefore said to them again, “Most certainly, I tell you, I am the sheep’s door.
(8) All who came before me are thieves and robbers, but the sheep didn’t listen to them.
(9) I am the door. If anyone enters in by me, he will be saved, and will go in and go out, and will find pasture.
(10) The thief only comes to steal, kill, and destroy. I came that they may have life, and may have it abundantly.
(11) I am the good shepherd. The good shepherd lays down his life for the sheep.
(12) He who is a hired hand, and not a shepherd, who doesn’t own the sheep, sees the wolf coming, leaves the sheep, and flees. The wolf snatches the sheep, and scatters them.
(13) The hired hand flees because he is a hired hand, and doesn’t care for the sheep.
(14) I am the good shepherd. I know my own, and I’m known by my own;
(15) even as the Father knows me, and I know the Father. I lay down my life for the sheep.