Why did Knights fight dragons?

It was considered training for combat.

A Bit More....It was also a form of entertainment, a place to prove/show off/practice/hone your skills and the the winners got lots of rewards!! A Bit more of more...it was very profitable sport and it was addicting as if u downed an opponent you could claim there weapons as a "ransom" and their shields, horse, weapons, etc were a knights most valuable possessions.