Every rule in Islam is based on our interest and avoiding us any harm. We do not know all the details of the reason but Allah (SWT) and the Prophet (SAWA) know the full reasons. We trust Allah and His Messenger and believe that dog is Najis because it is harmful for us to be in contact with dog.
Many medical scientists confirmed that many dangerous parasites can come to humans from contact with dog.
Journal of Medicine and Life stated : Dogs are a major reservoir for zoonotic infections.Dogs transmit several viral a bacterial diseases to humans.zoonotic diseases can be transmitted to humans by infected saliva aerosols,contaminated urine and feces and direct contact with the dog.