This is an extended metaphor for the Christian life without absolute surrender to God. You can get to the dune, to the pool, to the ocean, to the ice-rink but not really experience the views, the swimming, the gliding, unless you enter in through the narrow gate.
Which is absolute surrender to the will of God.
Absolute surrender is the way to live, leaving more and more of the business of our lives in God’s competent hands. An action, an act of will which we might need to go through several times in a day!
It is the narrow gate into life. It is like death to do it. Have I done it fully?
No, alas, not yet, but I do try to bring my life area by area, into sync with the will of God, just as I am decluttering my house, and bringing order into it, room by room, inch by inch.
I have resisted for years, but I now think, like most hard things, the sooner one gets it done with, the better. The sooner one can begin to enjoy life, to experience joy.