Maybe sheep are like lemmings — all going the same way, following along in a daze, unaware of dangers around them, oblivious to the paths they might take. Lemmings tend to move forward in a crowd, even to the point of tumbling over cliffs to their deaths.
For sheep, it is natural to follow a leader, yet they also tend to keep together. Sheep usually flee when threatened and only stand their ground when their egress is cut off.
A narrow gate is one which is obvious, but difficult to pass through especially if sheep are in a large mob — herd or flock. Many may not pass through; rather each sheep must find its own way through that narrow gate. Most sheep remain with the herd, meandering through the wide gate which is easy to pass through and appears safer, since the sheep remains in the mob.
But that wide gate — which is so easy to pass through — leads to destruction.
And the narrow gate — which is so lonely sometimes and so hard to find and so difficult to pass through — “leads to life and those who find it are few.”