From the perspective of halakha and also of experience, fasting and mourning share a great deal in common. On a basic level, both involve a somber attitude, a withdrawal from creature comforts, and a tendency to introspection. In terms of observance, there is a significant amount of overlap between the two domains. On major fasts (i.e., Tisha B'av and Yom Kippur) we observe prohibitions on sexual relations, the donning of leather footwear, washing and anointing, and all of these restrictions also apply (although in the case of washing, somewhat less stringently) to an individual who is sitting Shiva for a close relative.
That being said, the halakhic differences between mourning and fasting are equally noteworthy. One is not expected to fast during the period of Shiva - on the contrary, meals are regularly provided to the mourners. On the other hand, additional restrictions on mourners, such as the prohibitions to study Torah or to greet friends, for example, have no parallel whatsoever on Yom Kippur, the most intense fast day of the Jewish calendar. It goes without saying that no tragic occurrence is necessary to precipitate a fast - Yom Kippur is observed even during the best of times and has nothing to do with misfortune.
The fast of Tisha B'av illustrates the mourning-fasting dichotomy nicely, inasmuch as it is a hybrid of the two. On Tisha B'av - and, to a lesser extent, on all of the Rabbinical fasts - we are engaged in mourning the destruction of the Temple. From what we know of the laws of mourning, it would have been at least theoretically possible for the Prophets to institute a day of mourning for the Destruction without incorporating the element of fasting as well. However, for some reason they saw fit to merge fasting and mourning together in the observance of Tisha B'av. The halakhot of the Fast, some of which are derived from Yom Kippur and others from the laws of Shiva, bear witness to the dual nature of Tisha B'av.
So what is it exactly that creates the affinity between mourning and fasting? Why do they share common aspects while differing from one another in several key respects?
Mourning is a natural response to tragedy. When calamity strikes an individual, a family or a community, it tends to place the everyday pleasures we relish in a totally new perspective. We all remember how jarred we were in the aftermath of September 11th, how people (briefly) exhibited a diminished interest in the pursuit of physical and egoistic gratification. Indeed, we would have looked upon anyone preoccupied with his own agenda at that time as callous and arrogant - how could someone be so utterly insensitive to the import of what had befallen us, and simply go about life or business as usual?
This is the sense in which the laws of mourning mandate a withdrawal from physical pleasure. Proper deference to the significance of a loss, whether it be personal or national, is expressed in diminished pleasure-seeking. One who appreciates the seriousness of such a tragedy cannot possibly pursue enjoyments with the same zeal that he did previously. The trivial gratifications that used to draw him seem petty and meaningless when he is faced with the reality of his own mortality and vulnerability. The "finer things in life" do not possess the same allure to a broken soul, now bereft of a precious loved one.
A person who persists in his selfishness despite the endurance of calamity is viewed as morally reprehensible because he is deaf to the lessons that we expect such events to teach him. His enslavement to dreams of power and pleasure makes him utterly insensitive to the precariousness of his own existence and blinds him to the value of the existence of others. Just think of those who, in the wake of 9-11, immediately saw it as an opportunity to scam people in distress and to profit from their misfortune rather than an opportunity to reconsider what is truly important in life.
The mourning we observe on Tisha B'av, and the supreme importance attributed to it in our tradition, is consistent with these ideas. Being sensitive to the distance between the Jewish Nation and God, and focusing our minds on the horrific events that have befallen us in our Exile, must reduce the bounce in our step at least somewhat. Certainly the process should inspire us to withdraw a bit from the comforts and gratifications that usually beckon to us - whether they be physical or intellectual (as in the case of Torah study) - and to give serious thought to the Divine message that is embodied in our history and its implications for us.
One who views the solemn prayers and devotions of Tisha B'av as an unnecessary interference with his lifestyle that cannot be countenanced is surely insensitive to the tragedies that have scarred the Jewish nation over many centuries of exile. He is more interested in the potential for enjoyment in the here-and-now than in reflecting upon the vulnerability and persecution of the Jewish people witnessed to us by history. Why weaken his zest for life by reminding him of these harsh, depressing and possibly even frightening realities?
At the same time, however, Tisha B'av is not only a day of mourning - it is also a Taanit, a day of affliction and fasting. Whereas in mourning we withdraw from the pleasures of this world because their appeal naturally seems to fade in the face of tragedy, in fasting we deliberately detach ourselves from the pursuit of pleasure in order to harness our energies for more transcendent objectives.
A fast need not be connected to any depressing event. It can be a self-imposed regimen for the purpose of individual spiritual development or a nationally observed holiday like Yom Kippur. The purpose of a fast is to enable us to rise above the overwhelming pull of our instincts and to subject them, and our personalities as a whole, to serious analysis and correction. In this way our minds reassert themselves as the governing agents of our action, and our bodily desires are put firmly in check as we evaluate our lives without the comforting distraction of food and drink.
It is now clear why both mourning and fasting involve diminished physical gratification. In the case of mourning, this is a healthy reaction to tragedy and loss. Mourning would not typically lead us to neglect basic necessities like eating and drinking, since even an emotionally bruised psyche requires nourishment.
On the other hand, in the case of fasting, we are engaged in a conscious "affliction" of ourselves. We are actively attempting to remove distractions that might interfere with our process of repentance by sapping our psychological energy or dulling the clarity of our thought. Unlike the in the case of mourning where the change in our relationship with pleasures is a byproduct of our preoccupation with something more important, on a fast day the state of physical deprivation is part of our goal - it is a preparation and a springboard for meaningful introspection and personal transformation. Hence, even eating, drinking and other relatively harmless forms of pleasure are strictly prohibited on major fasts.
In light of this analysis, we have a clearer grasp of the structure and themes of Tisha B'av. The objective of reflection upon the tragic chapters of our history engenders sincere mourning. This, however, is insufficient. Preoccupation with tragedy, however intense, is always short-lived. People devastated by loss can, with the passage of time, get over it and return to life with the same zest for enjoyment they previously possessed. Once the proverbial storm has passed and its effects begin to wear off, such individuals can and do emerge from the experience of loss fundamentally unchanged.
For this reason, Tisha B'av requires not only mourning, but also Taanit - affliction and fasting. This ensures that our meditation on the tragic elements of Jewish history has an impact on our lives and is not in vain.
By linking the mourning of Tisha B'av with fasting, our Sages taught us that the destruction of the Temple and our turbulent existence in Exile are not just depressing realities we are compelled to live with, discuss endlessly and cry about helplessly each year. On the contrary, our acknowledgment of the significance of these episodes in our past should move us to reconsider the direction of our lives and to commit ourselves more sincerely and earnestly to fulfilling our covenant with Hashem. In this way our observance of Tisha B'av will serve to bring us one step closer to the ultimate redemption for which we pray.