Your dog could have a runny nose for a number of reason with some being more serious than others. If the nasal discharge is clear then it is likely that your dog could be suffering from allergies. Just like humans dogs can be allergic to pollen, foods and even humans.
Your dog's runny nose might be due to something causing a blockage. You may be able to see the cause and be able to carefully remove it, or it may sort itself out. If the discharge is cream/yellow/green in colour then your dog probably has an infection and you will need to contact your vet as the issue could potentially be serious.