Why am I frightened of dogs?

I don't know.  I don't know because I do not have a glimpse into your experiences.  To answer as the question is asked without supporting information, it is likely do to an unpleasant experience you have had with a dog.  When we experience an unpleasant or frightening experience in which we felt unsafe and possibly harmed, our brains will code this information so that upon a similar encounter we react appropriately to remove ourselves from the danger.  

If you are seeking to not be fearful of dogs, there is really good news!  It's 100% treatable.  You can find yourself petting and walking a canine in just a few short weeks!