Why did the Boers resist British rule?
I would argue that the Boers resisted British rule on nationalist grounds and on the grounds of what we would now call self-determination.
The Boers were not British. "Boer" is the Dutch word for "farmer." They were Dutch with some who were French or German. They came to what is now South Africa when the area of the Cape of Good Hope was controlled by the Dutch East India Company. The Boers generally felt loyalty to the Netherlands and to Protestantism as opposed to England. As England came to take control of the area, the Boers resisted. They did this because they felt that they were Dutch and that they should not be ruled by English people. In addition, they felt that they had been there first (before any other whites, at least) and that that gave them the right to the area.
So the Boers resisted the British largely because they were Dutch and didn't want to be ruled by English people who were Anglican.
adding on to what #1 answered you the boers were a group of people who were aware of their identity, so it was rather difficult for them to accept british rule with its new culture and means of survial, basicallly the had to stand their ground in fighting to prevent 'colonisation'