The following causes of animal-unfriendly behaviour can be found (please e-mail us if you can mention more):
Traditions that can sometimes be traced back a long time, such as hunting and ritual slaughter.
Ideology, such as the notion that mankind can and should rule animals.
Religion, for example Mithraism. In Roman times this religion was an important rival of Christianity. Practitioners of Mithraism used to sacrifice a bull every year, and because of the rivalry between these two religions, the Christians decided to mix the imagery of the devil with that of the bull. It is possible that bull-fighting finds its origin in the ritual slaughter of bulls in the times of Mithraism. It was said that a bull's blood would purge one of one's sins, f.e. the "feast" of Farra Do Boi in Brazil.
Ignorance, be it or not on purpose.
Arrogance, inability of man to accept the equal rights of other living souls.
Economic gain, sometimes pushed to extremes.
The way people treat animals reflects their beliefs, values, knowledge and the importance of the animal for them, either financially, socially or morally.