I understand that the 5 forms of inuy ("affliction) that occur on Yom Kippur also applies to Tish'a B'Av. One of these is the prohibition to wear leather shoes.
I viewed this M.Y. answer which offers some explanation why wearing shoes is prohibited. I'm inferring that the original idea was to walk barefoot. This, indeed, would be a form of affliction.
However, currently, we substitute the leather shoes with sneakers. To me, and, I think, most people, find that sneakers are actually more comfortable than leather shoes. They are light weight, cooler, and many have "vents". So, if the sneakers are actually more comfortable, how would wearing them be a form of affliction? It sounds like it's accomplishing the opposite.