There are a few reasons why you could be experiencing this, including: -Genetics (or random genetic mutations) -Sun exposure on one side of the face (for example when driving a truck for 10 -15 years and always having the left side exposed to the sun) -Sleeping on one side and having more rubbing and tugging occur on that side -Hair styles that would pull more on one side than the other -Being less thorough washing one side of the scalp and having more product residue -Having kids who like to pull your hair and who somehow prefer one side over the other -Some sort of rubbing against the shoulders or back