Leather shoes on Tisha B'Av
Why are leather shoes of all things prohibited on Tisha B'Av? RSZA has a fascinating answer based on the של"ה.
The Beracha of שעשה לי כל צרכי goes on shoes (in fact according to the Rambam and Shulchan Aruch you make it after putting on your shoes). The obvious question is why? Are shoes your only need? A person who has shoes has no other needs? He has everything? The של"ה explains that leather shoes represent man's dominance/rule over the world. The fact that man can kill an animal and wear it's skin to protect his feet is the ultimate expression of dominance. Therefore leather shoes represent man's dominance over teh world and the fact that the world provides all of man's needs.
Based on this, we can explain why there is an issur to wear shoes on Tisha B'Av, Yom Kippur an Avel, in the Bei Hamikdash. All of these are times/places where man needs to show humility before Hashem and therefore to wear leather shoes the symbol of man's dominance would be inappropriate.