A young couple gets married. They are finally permitted to touch each other and express their strong emotions towards one another. Is it conceivable that precisely at that stage, they are instructed to be completely physically separated from each other, for at least twelve days every month? How can we possibly explain and understand this phenomenon?
Shir Hashirim Rabba (Parasha 7,3) poses a similar question: "Is there an iron wall separating between the couple? Or perhaps, a snake or a scorpion bit the husband and stopped him from coming close to his wife?" The answer, and the only possible answer, to such an unthinkable predicament is: the words of the Torah. Only the Torah has the compelling power to stop their powerful desire to be physically united.
The answer to why "family purity," is a G-dly command is that we do it because the Holy One blessed be He, in His infinite wisdom instructed us so. It is neither an iron wall, nor the bite of a snake or a scorpion which stops us, but rather our free choice to follow G-d's will, which is likened to a fence made of roses. Anyone who chooses to go forth may do so, the decision to refrain from passing it is ours too.
Numerous explanations are suggested for why we have the laws of Family Purity. One of them, given by Ribbi Meir 'a"h, (Niddah 31a) may be described by the expression: "familiarity breeds contempt". Separating from each other creates an effect of longing and anticipation to be together again and eliminates the development of boredom and contempt. This separation endears the wife to her husband, with the same intensity as the husband felt towards her on the night of their wedding.