Nasal flaring is seen mostly in infants and younger children. Any condition that causes difficulty breathing can cause nasal flaring. While many causes of nasal flaring are not serious, some can be life threatening.
In young infants, nasal flaring can be a very important symptom of respiratory distress.

Asthma:
Nasal flaring is common in people suffering from acute asthma. It may occur along with other common asthma symptoms, such as wheezing, tightness of the chest, and shortness of breath. Asthma can be triggered by a number of stimuli, ranging from animals to dust, mold, and pollen.

Bacterial and Viral Infections:
You may notice your nostrils flaring if you are suffering from a severe infection, such as influenza (flu). It is most commonly seen in people with serious respiratory conditions, such as pneumonia and bronchiolitis. Another common cause is croup, an inflammation of the larynx and trachea in children, which is associated with infection.

Epiglottitis:
Epiglottitis is inflammation of the tissue covering the trachea (windpipe). It is rare now because of childhood immunization against the bacteria that causes it (H.Influenzae type B). Epiglottitis was once most commonly seen in children aged two to six. It would be rare for an adult to develop the disease.