Because narrow is the gate, and narrow is the way, which leads unto life, and few there be that find it.
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Theophylact of Ochrid
AD 1107
The word "how" expresses the Lord’s wonderment, as if He were saying, "Alas, how narrow it is!" But how is it that the Lord says on another occasion, "My yoke is light" (Mt. 11:30). It is light on account of the future rewards.