Why does a person get lupus?

Lupus causes. Normally, during the early development of the immune system, blood cells the could potentially harm the body, by making antibodies that bind to "self" antigens are destroyed. Other times these cells are prevented from harming us because "suppressor" cells keep them under control and inactive. The loss of this immune control can cause an auto-immune disease, why this occurs is unknown.
Lupus. Lupus is an autoimmune disease. Difficult to say what triggers but the antibodies in your immune system go haywire and start recognizing "self" as foreign and attacks.