The narrow gate is Christ Himself, the one way that almost no one expects: grace, not effort. The wide road is every self-made path to God. Narrow doesn't mean God is stingy. It means there is exactly one door, and it's a Person.
The Grace Answer
This image gets preached as a warning to try harder: the narrow way is the disciplined, sold-out life, and most people are too lazy or too worldly to walk it. Read that way, the gate is narrow because the standard is high and few can maintain it. That reading quietly makes salvation about your performance, which is exactly what Jesus is not saying.
Look at what the narrow gate actually is. A few chapters later Jesus says plainly, “I am the door. If anyone enters by Me, he will be saved.” And, “I am the way, the truth, and the life. No one comes to the Father except through Me.” The gate is not a lifestyle you achieve. The gate is a Person you trust. It is narrow because there is exactly one of Him, and He is the only way to the Father.
Why so few find it
The wide road is crowded because it holds every other path: every religion of self-improvement, every attempt to be good enough, every version of earning your way to God. Those roads feel natural. Human pride loves a path it can walk on its own merits. The narrow gate offends that pride, because you cannot earn your way through it. You can only receive.
So narrow does not mean God is reluctant or stingy with heaven. The invitation goes out to everyone: whoever will may come. It means there is one door, and it is grace rather than effort. Ironically, the narrow gate is the easy one to walk through, because Christ already did all the work. It is only narrow to the person still insisting on getting there their own way. So the question the passage presses is not are you trying hard enough, but have you come through the one door. Everyone who has is already on the road to life, no matter how faltering the steps feel.