The most common cause for a runny nose is the common cold or flu. This typically increase in severity over 2 to 3 days often associated with a cough or fever. It runs its course and recovery is expected within a week.
If symptoms persist beyond a week, sinusitis will need to be excluded especially if the discharge is thick and discoloured.
If the discharge is clear and there are morning symptoms of sneezing and congested nose, the most likely cause is allergy.
Allergy can be triggered by the environment or diet. There is an increase risk of asthma if the allergy is not well controlled. With proper diagnosis and treatment, cure is possible for allergy.