The contraction of the facial muscles used when you laugh or smile, or when you scrunch up your nose, creates fine lines that appear on either side of your nose. These bunny lines can become permanent and eventually deeper wrinkles as the skin loses its elasticity due to ageing, sun damage and other factors.
Another common cause for bunny lines is botulinum A toxin. When certain facial muscles are relaxed by botulinum A toxin injections, others will take their place to convey certain facial expressions. These muscles will become hyperactive in order to compensate for the inactivity of the muscles injected with botulinum A toxin, causing the creation of these wrinkles on the sides of the nose.