The fast of shiva asar b'Tammuz begins the mourning period known as the 'Three Weeks'. On this day, the walls of Jerusalem were broken down, and three weeks later, the Beis HaMikdash was destroyed. It is instructive to delve deeper into the purpose of fasting, in order to enter into this sad period with the appropriate state of mind.
The Ben Ish Chai zt"l writes that there are two main purposes of fasting. The first reason is fairly apparent - that fasting diverts a person from physical involvement so that he can focus on more spiritual matters. His second reason is a little less obvious; he explains that when a person fasts, he feels hungry and endures considerable discomfort. By placing himself in such a predicament, he can come to a greater appreciation of the constant hunger and discomfort that a poor person faces throughout his life. This increased awareness will heighten his feelings of pity for the poor man's situation, and will motivate him to offer greater assistance to his unfortunate fellow.
The Ben Ish Chai applies this explanation to clarify an enigmatic gemara in Brachos. The gemara tells us, 'the reward for fasting is charity'. He explains that the gemara is telling us that the consequence of fasting is that a person will give more charity. His very act of fasting will cause him to be more caring about the poor people whom he constantly encounters, and accordingly he will want to help them to a greater degree. It is possible to add that fasting has another benefit related to doing chesed with those less fortunate than oneself. As well as causing a person to give more abundantly, it enables him to improve his giving in a qualitative fashion. By temporarily placing oneself in a situation similar to that of the poor person, he is able to show a far greater sense of understanding for his fellow's desperate situation. When the giver shows that he truly empathizes with the receiver, then the act of giving constitutes a far greater act of chesed.
The Sifsei Chaim explains an Avos d’Rebbi Nosson in this vein. It says, “one should greet every man with a friendly countenance… if a person gives to his friend all the gifts in the world, but his face is sullen, it is considered as if he gave nothing. But one who greets his fellow with a friendly countenance, even if he gave him no gifts, it is considered as if he gave him all the best gifts in the world.” The Sifsei Chaim writes that what people want more than anything is for others to show an interest in and care about them. A gift is merely an indication that the giver thought about the needs of his fellow and how he could give him joy. However, without an accompanying show of warmth, the main purpose of the gift is lost, because the person does not feel as if he is being genuinely cared about. In contrast, when a person is friendly to his fellow even without giving any gifts, then he is providing him with his main need, the desire to feel cared about. A person who gives charity with a friendly attitude is giving much more than money, he is nourishing the poor man with a sense of importance by showing that he is cared about. Similarly, when the poor man feels that his fellow truly relates to his pain, then he feels a great deal of comfort that someone truly understands and cares about his plight.
Rav Shach zt"l excelled in doing chesed by showing an understanding of his fellow's challenges. On one occasion he heard about a widower who was depressed to the point that he stopped functioning. Rav Shach decided to visit the man in an attempt to bring him out of his depression. Receiving no response to his knock, he let himself in, and found the man lying motionless on the couch. "I know what you are going through," he said to the man. "I'm also a widower. My world is dark and I have no joy." The man's eyes lit up for the first time in months - this encounter was the catalyst of the man's resumption of a normal life. What was Rav Shach's secret? By stressing that he too experienced the feelings of losing a spouse, he showed the man that somebody truly understood his pain.
In this instance, the giver had first-hand experience of the receiver's situation. When one is fortunate enough not to endure the same difficulty, he must adapt the lesson of the Ben Ish Chai and try to somehow place himself in a state where he can somewhat relate to his fellow's plight. Rav Noach Orlowek Shlita applies this lesson to help people empathize with those who lose family members through some kind of tragedy. He suggests that when one hears about such a tragedy he should spend a short time thinking about how he would feel if this happened to someone close to him. In this way, one can develop a far greater sense of empathy with those who suffer tragedy.
This lesson of fasting is particularly relevant to the 'Three Weeks' Chazal say that the Second Beis HaMikdash was destroyed because of failings in the area of bein adam lechaveiro. A failure to empathize with the situation of one's fellow is one of the main causes for such flaws. It is far easier for a person to harm others when he has no sensitivity to the pain that he causes them. The Ben Ish Chai teaches us that fasting can be an effective way of eroding one's apathy for his fellow Jews. May we utilize the fast of shiva asar b'Tammuz to improve our conduct in bein adam lechaveiro.
 Taanis, 26a-b.
 Brachos, 6b, See Rashi and Maharsha for their explanations of what this means.
 Avos d’Rebbi Nosson, Ch.13.
 Sifsei Chaim, Moadim, 3rd Chelek, p.275, footnote 11.
 Kaplan, 'Major Impact', p.55-6.
 See Yoma, 9b, Bava Metsia, 30b.
 Psychologists observe that criminals are able to cause others great pain because they are totally immune to the pain that their victims endure. Accordingly, one of the most successful forms of therapy is making the criminals relive their crimes from the perspective of their victims. In this way, they develop some idea of the tremendous pain that they have caused innocent people.