okay, for the first time in writing, my "separation makes the heart grow fonder" idea!:
I've always been a fan of the saying that "separation makes the heart grow fonder". In other words in any relationship, being available too often, being ever-present, is not beneficial but is in fact detrimental to a relationship. For with anything comes boredom. Even the object of your romantic desire can quickly become "boring" to you. But if you're forced to be away from them for a while, upon their return comes a rejuvenated closeness and intimacy that was not possible before. Which is the rationalization given for the Purity Laws as they apply to menstruation.
This principle doesn't only apply to a relationships though. It's applicable to many other aspects of life. In all things: if someone experiences a continuous orgy of pleasure, after a while it is not within his ability to experience pleasure. Whether it be in food, sexuality, sleep, sport, honor; anything which Man finds pleasing. Simply put: whatever you 'have', you don't 'want' anymore. You want what you don't have. It's obvious really. The glutton does not enjoy a good meal as much as the pauper. A licentious man does not enjoy sexuality as much as the celibate man. The absence of a thing, in a sense, is the only way to really have it! Absence IS presence. Abstinence IS sexuality. Hunger IS food. Pain IS pleasure! They are points of contrast and comparison. This idea has a central role in thought-processes, and I apply it to many aspects of life.
I once happened upon a blog that criticized Rabbi Avigdor Miller's asceticism. For example, after his death it was revealed that Rav Miller slept on a board, not a bed, he would constantly stop eating while he ate, and many other such practices. This blog called Rav Miller's chamber a torture chamber. I objected. I saw he was an atheist so I told him that not only did causing yourself privation make sense in Judaic terms, but it made sense in general as well. I told him about a television program I once saw, which told the story of a group of German adventure seekers who climbed parts of the alps that are usually traversed by no one. At the end of it they were starving and half dead. When one of them was asked by reporters why he did it, he said that when he goes back to Berlin now and has a beer, it'll be the best beer he ever had! He understood that pleasure does not come without privation.
Another aspect of life I've been associating this principle with recently is the senses of entitlement v.s. appreciation. Here in the my hometown of Brooklyn, more than anywhere else I've seen, and especially in the realm of dating (as is no wonder), I've come across people who have always had far more than I, and yet grew up in an absent-minded sense of great entitlement. Especially the daughters of "our community", as they're provided for more than the boys. Especially the fairer ones. They date as if they're trying on clothes. And they'll only take what they feel entitled to, which is probably part of the "shidduch crisis". I think that had these picky girls known loneliness, in it's severer forms, they would be less picky. Again, it is privation that humbles the soul, and brings it from haughtiness to humility, and therefore, to happiness.