Why should you not wear earring in only one ear?

Have you seen any girl who is wearing one earring in either left or right? how are they? are they all happy and satisfied? No, most of the time we see such people, they are tensed. So how is tension related to wearing earring in only one ear? - Tension is probably because they lost their 1 earring!
And have you seen others who are not so tensed? It's probably because they are not wearing other one due to lack of money! Conclusion: wearing earring on both sides reduces your stress.

Ok, I was joking in above paragraph. Now We will try to see is there any scientific relation to belief that earrings should be worn in pair? If we take reference of  acupuncture, there are many end point of nerves in our ear which is directly related to brain, and pressure at certain point affects the part associated with it. So if we wear only one earring, it will only trigger response on one side and other side remains undisturbed! This may cause some imbalance.

Ear piercing for weight loss:
According to some sources, the outer ear and the abdominal organs are right next to each other during fetal development in the womb, and the physical connection between these body parts remains via nerve pathways even after they are no longer in close proximity. In this explanation, stimulating a part of the external ear also stimulates the abdominal organs, including the stomach. Other practitioners claim that piercing this particular area of the ear releases endorphins -- chemicals released by the brain during pleasurable activities that make you feel good. And because the staple stays in the ear, the endorphin effect is maintained, reducing a person's stress and anxiety levels, which for many people are a big part of the reason they over-eat.