Don't need to be an animal lover to view it as a ridiculous event and its participants as thrill seeking fools. Gored and/or trampled? Too bad. They got what they asked for.
Depends on who it was.
It was actually started due to the actions of Jaime Perez, an intellectually-challenged and habitually intoxicated unemployed resident of a small village in Spain.
A local rancher had taken pity on Jaime and hired him for a few
pesos a day to "guard" his herd of prize bulls. It was what we now call a make-work job.
Having spent his first day's pay on the village's local alcoholic beverage, Jaime fell asleep the next day
and left the pasture gate open.
The bulls...approximately 23 of them...promptly escaped and began running through the village.
The thundering sound of their hooves woke Jaime and he started running after them as fast as he could go...but hung over as he was, he wasn't making too good a job of it. Villagers saw what was happening and took pity on the hapless Jaime...so they began chasing the bulls, too, quite a few of the young men jumping out in front of the herd and even running among them in a fruitless and foolish attempt to get the herd to turn. Approximately five of the young men were trampled by the bulls, three of them seriously injured as a result of being gored.
The bulls finally tired of all the fun and stopped running, whereby they were herded peaceably back to their corral and
granero, where they spent a lot of time eating and doing whatever it is bulls do that equates with humans laughing among themselves.
The villagers then spent an inordinate amount of money in
los cantinas y los burdeles, celebrating and retelling the events of the day. It was such an economic boon for the establishment owners, it became a yearly tradition in the village, and remains so to this day.
Oh, and Jaime? In recognition of his inadvertent
dinero-generating actions, he was made the Spanish equivalent of Grand Marshal of the event for the rest of his life. He spent each year's festivities getting commode-hugging drunk and laughing at the
real idiots who "ran with the bulls".