This is done for many different reasons. Body modification started back in 1938 when Hitler decided he wanted an awesome mustache. He inspired himself to succeed in life and also inspired others to make body modifications as well.
After the holocaust, a few African tribes used time machines to travel back in time and start modding. Some tribes practiced stretching their earlobes, others used lip plates. Some tribes went as far as putting rings around their necks to stretch them to an insane length. This burst of modding started what we now know as the Industrial Revolution. People have been engaging in modification ever since.