QUESTION:
For most of פסח we are very careful that our מצות do not become wet. This is because we are concerned that perhaps there might be some raw flour that didn’t mix well with the dough and didn’t bake, which might חס ושלום become חמץ if it comes in contact with water at this point.
However, on אחרון של פסח, the Rebbeim were not concerned about their מצה getting wet, and were actually particular to intentionally wet their מצה in every dish.
Why?
ANSWER:
מצה represents humility, while חמץ represents arrogance and haughtiness.
We find that on פסח, eating, benefiting from, and even owning חמץ is forbidden. However, on שבועות, in the בית המקדש, the offering of the שתי הלחם, which was חמץ, was obligatory.
This is because on פסח, when one is only beginning his spiritual עבודה in leaving מצרים, there is a danger that a person’s ego can lead to negative things. (Since he is important in his own eyes, he might allow himself to do whatever he wants). So חמץ, like the ego, is absolutely forbidden.
However, on שבועות, after seven weeks of ספירת העומר, when one refined all of his מדות in preparation for מתן תורה, there is no such concern. At this point, the only sense of self that he has is not his negative ego, but his positive pride in serving Hashem. Now he can - and therefore must - utilize this as well. So חמץ, representing that pride, becomes obligatory.
On אחרון של פסח, having completed one full week of ספירת העומר, the transformation has begun; still, one is not yet totally transformed, so חמץ is not yet obligatory. However, with one complete week, one has already reached a level where the risk of the ego is not as strong as it was, and therefore one may allow themselves to wet their מצה. And since we can, we must.
Further discussion:
This explains eating שרוי' after 7 days of ספירה. What’s the connection to אחרון של פסח, specifically? (See אות יו"ד in the שיחה)
