“One may ask: Is not God’s mercy without end?
The answer, of course, lies in the fact that the giving does not remain in the realm of the abstract.
When Chesed becomes connected with the specific needs of those who receive grace, each according to his place in the value system of the World of Action, it can no longer bestow without measure.
The abundance can become a curse, like flood when the blessed rain keeps on falling.
This, in the Kabbalah, is the ‘breaking of the vessels,’ what occurs when the Light exceeds the capacity to receive it.
Does not everyone experience the smallness of the capacity to absorb pleasure?
Do we not cry out ‘Enough or I bust!’ even though we feel that our desire is measureless and our need immense?
For man is a fragile vessel.
Thus the very finitude of the vessel restricts the downpour of abundance, and divine Grace has to restrain itself (with the aid of Gevurah).
If one teaches something that is too much for the pupil, one has really not given him anything at all.”
From “Returning Light and Divine Laughter”, p. 171-172 in In the Beginning by Rabbi Adin Steinsaltz