I have a droopy eyebrow and want to know what the cause is. How does this happen to only one eye?

You need to send your pictures so that we can give you a response that makes sense.
There are too many possible causes. You could benefit from a brow lift, Botox, Then, the fact that you state that this is recent - go see a doctor. This is not the venue for such an acute problem for someone your age.

Twilight21,
Most eyebrows naturally droop or descend with age. The skin and connective tissue around the eyes and eyebrows weaken, which makes them drop. The side of the eyebrows tend to drop more than the part of the brow near the nose.
You are too young for age to be a cause. One other common cause of droopy eyebrows is congenital or genetic. Do your siblings, parents, or other family members have droopy eyebrows?
A more concerning cause is weakening of the facial nerve. The facial nerve controls the muscles of the face. When this nerve is weak, the muscles it supplies also become weak. Weak facial muscles result in droopy eyebrows. Other parts of your face may appear droopy too. Sometimes facial nerve weakness (ie. facial paralysis) may be associated with strange sensations of the face or taste. There are numerous causes of facial paralysis.
Speak with your primary care physician to perform a comprehensive examination, who may then refer you to a specialist for further testing.