It is interesting to study how the idea appeared. First of all, the dragon is a magical creature superior to many of other creatures. The slayer is usually human. But how a simple weak (in comparison) human can defeat such an enemy? Dragonslayer is rarely a simple human. A human overcoming such a great enemy (not always an enemy though), is a symbol showing that human is capable of everything.
(Glad to see your opinions and additions)
(Hobbit:There and back again) Bard - killed the dragon named Smaug with the black arrow. Bard saved Esgaroth from destruction.
(The Ring of Nibelungs ) Sigfrid – killed the dragon and washed in his blood, making himself invulnerable.
(Please continue the list with your examples. I'm going to update the entry with your additions)