The light of the sun exists only because of its separation from the sun.
The nearer to the sun anything gets, the less does it possess an independent reality.
In human terms, one could say, "No man shall look upon Me and live."
No one can approach God beyond a certain distance.
And it is not a matter of physical capacity; the impossibility is a function of the very essence of things.
One cannot remain a human being and see the face of the Divine.
For as soon as I approach God, I cease to be something separate.
Neither terror nor fearfulness is involved.
It is simply not possible to be in two places at the same time.
Our being human depends on this distance from God.
To give another example: in any play of light and shadow, the distinctiveness of form depends on maintaining a minimal distance from the source of light; as soon as one gets too close, the whole thing is dissolved and made unfeasible.
The sin of the sons of Aaron–who approached the Ark of the Covenant and died–was that they crossed a certain border, beyond which one may not venture and live.
This is connected incidentally, with a basic idea of Judaism, that one has to be careful not to cross this boundary between man and God, because if one does, one cannot return.
–Rabbi Adin Steinsaltz
From "The Nullification of Reality" in The Sustaining Utterance by Rabbi Adin Steinsaltz