As stated in the title from after the Great trek in 1836 the Boers wanted to govern themselves and were dissatisfied with British rule. Why did the British keep on insisting on ruling over people who went to such lengths to escape their rule?
The British were in South Africa both for its resources, and to make possible a railway running north-south between Egypt and South Africa. The Boer heartland was in the north-east part of the country, centred on Johannesburg. The British were concentrated in Capetown, more accessible by sea, so although the British politically controlled South Africa there was some physical separation between the parties.
To allow the Boers to self-govern would both bring under threat British objectives, and also restrict their ability to oversee the difficult relationships between the Boers and the native Africans. As Germany emerged in the late 19th Century as an industrial, the therefore colonial, rival, the threat to British interests increased.
These were the prevailing factors on the eve of the Boer War.