It is "me gustan" because really you are saying "The dogs are pleasing to me". You are not saying "I like the dogs", although that is how we normally say it in english. So, the dogs are the subject, and they determine the conjugation of the verb, gustar, in the third person plural. If you wanted to say it in the preterit, it would be "me gustaron los perros". "The dogs pleased me (until one bit me, maybe?)"