During the time when a woman is niddah, she is forebidden to her husband. Clearly this is a very strong metaphor of exile, of galut. The same couple who was reunited as one flesh must now be separated and distanced, could this be any closer to a text-book understanding of HaShem's relationship to Israel in exile? It can be a very depressing time for a couple, and it certainly is a very serious time as kalut rosh (behaving lightheartedly (or light headedly more literally)) is forebidden.
It goes without saying that it should not be a depressing time, as maintaining a happy demeanor is at all times one of the greatest mitzwoth. The Ariz"l explained once that all of his accomplishments were due to his great joy in performing HaShem's will. So, at least in theory it shouldn't be a depressing time, but theories only work on the head, they don't hold much sway on the heart.
The Baal HaTanya explains that when one is very distant from HaShem and feels no connection, one of the best ways to open our hearts to prayer is to beg HaShem to have mercy on our pour innocent Neshamah, she, who is so far from her father's house, she who has done nothing wrong, who has been an innocent bystander in all of our wayward journeys.
From this we can stretch our horizons and applications of metaphors just a little, and understand something I believe I've mentioned in the past. That in the time of niddah, the husband has the unique ability to awaken tremendous rachamim on the nation of Israel. For he can connect in the most simple way to HaShem, who is exiled from Israel in her period of niddah. (aka galut)
Now that we have begun to understand the Baal Shem Tov's explanation of getting past the physicality and in touch with the essence via our dreams, we can delve to a new level of understanding niddah.
During niddah the husband and wife are forebidden to one another physically. In such a state, they can begin the avodah described by the Baal Shem Tov in understanding and connecting to the root source of their desires. The root source of their desires being the revellation of godliness in their spouse. This connection, when correctly recognized and acknowledged in thought and understanding, leads to a very deep spiritual unification in their dreams.
This is a closeness of spirit that was never possible in the same way when they were permitted to one another. It is only when they are forebidden uniquely to one another that they can aspire to connect their intimate desires on a purely spiritual level.
In this light it is amazing to understand that physically the most fertile moment in their relationship is the end of the niddah period when they are in such close connection spiritually and they are finally brought together in physical union as well.
Here, we can peek at the depth of the mitzwoth and their underlying structure. HaShem's ways are truly complete.(תורת השם תמימה) The multiple levels of closeness and distance, and the interaction of these forces, play a symphony of unification that allows two parts of a single soul to momentarily reconnect with such fusion as to actually taste HaShem. (טעמו וראו כי טוב השם)
From this level of specificity, (פרט) we can connect back to the whole (כלל) and see how the progression of history is a similar interplay of centrifugal and centripetal forces that in concert forge the union between HaShem and klal Yisrael.
Hopefully we can understand perhaps a little of our minor role in this universal drama through our intramarital dynamics, and find joy and inspiration in fulfilling our part.