Why is it fitting for the s’chach to leave us an opening toward the sky?
The s’chach is not an ordinary ceiling. The Shulchan Aruch notes that the usual way is for the covering to be light enough that the large stars can be seen through it, while also ruling that a sukkah covered densely like a house remains valid (Orach Chaim 631:3). The point is therefore not to invent a condition of validity, but to notice the experience the sukkah creates.
For most of the year we live beneath ceilings we have built for ourselves, and not only ceilings of concrete. We live beneath habits, fears and assumptions about what is possible and impossible. Yom Kippur may open the heart for a moment; Sukkot asks us not to close it immediately.
A person needs a home, but also an opening toward something larger than himself. Life can become so organized that it leaves no room for wonder, faith, surprise or change.
The sukkah reminds us that even a stable and responsible life should leave one opening toward the sky, so that our security does not become a ceiling that hides everything above us.