In this scene, the main character (Lewis himself) is talking to the Spirit who came to try and persuade him to go into the mountains - it is his hero, George MacDonald. Lewis is asking MacDonald why all the Ghosts must come to Heaven, and then be persuaded. If the Spirits wanted them to be happy, why didn't they go down into Hell to rescue them? MacDonald said that the Spirits actually come quite far from the mountains to rescue Ghosts - their purpose is to journey further and further into the mountains, so when they come back for the Ghosts, they are coming immeasurable distances on the slim chance of saving one person.
Lewis: But what of all the poor Ghosts who never get into the onmibus at all?
MacDonald: Everyone who wishes it does. Never fear. There are only two kinds of people in the end: those who say to God, "Thy will be done," and those to whom God says, in the end, "Thy will be done." All that are in Hell, choose it. Without that self-choice there could be no Hell. No soul that seriously and constantly desires joy will ever miss it. Those who seek find. To those who knock it is opened.
I can see how some people could read this and think that God is being mean-spirited, or doesn't care if we go to Hell or not. But really, isn't this what He does on a daily basis already? We constantly make our own choices and he will never stop that - so if we choose that we will be happier in Hell, living our sin, then that's what He'll let us do.
This so logically states the reason - and the goodness - that comes from having our agency. It's beautiful.