Board Thread:General Discussion/@comment-11462477-20130713223417/@comment-14222126-20130807070606

The way I understand it is that their are two ways to learn a Thu'um.

One is by Natural ability gifted by Kynareth/Akatosh (by absorbing a dragons soul), and the other is by extensive study as mastered and taught by Paarthunax/JWindcaller. Seeing that dragon shouts is a power that was made accessible to mortals, (much like magic) they are able to tap into the understanding of that power by meditation and reflection.

And yes, dragonrend was created by mortals in defiance of the dragons. The dragonborn is able to easily tap into it's power as he is both naturally gifted and a mortal.