There are many causes of leukorrhea, some of them are normal and some are abnormal. Normally, leukorrhea occurs because of hormone changes, for example before a girl has menses for the very first time. It sometimes also happens during certain days of a menstrual cycle ( because of hormone changes). Sexual excitement would also cause leukorrhea. These types of leukorrhea are considered normal, and will be thin and white in appearance, without any noticeable smell.
Main cause of abnormal leukorrhea is infection of female reproductive organs. Sexually transmitted diseases also cause abnormal leukorrhea ( however, you are unmarried so I am assuming that you are not sexually active). Leukorrhea associated with infections is generally thick and yellow in appearance, and has a bad smell. Another common abnormal leukorrhea would be the yeast infection which will cause cheese like appearance of thick white leukorrhea, with smell.
Abnormal leucorrhea needs proper treatment with antibiotics. If not treated properly, it may lead to PID ( Pelvic Inflammatory Disease) and sometimes difficulty with future childbirth. If your leukorrhea appears to be abnormal then you should get checked by your gynecologist.