So... I've heard about wardens and oaths, including from Alistair, but does anyone actually remember being asked to swear an oath? Because I sure don't (and at least one of my Wardens was a very reluctant recruit, she d**n sure wouldn't have sworn one
.
The Joining is an oath.
“Join us brothers and sisters.
Join us in the shadows where we stand vigilant.
Join us as we carry the duty that can not be forsworn.
And should you perish, know that your sacrifice will not be forgotten.
And that one day we shall join you.”
You are pretty much swearing to hunt darskpawn until one day you fall on some hurlock's nasty taint-greased sword... if you're lucky. If you're not, enjoy your Calling. 
But yeah, even if your Warden wasn't keen on joining, people do desert. Anders swore that oath virtually at gunpoint, and ran tail outta there the first opportunity he got. My own Cousland resisted her "recruitment" until Duncan had to drop the whole nice guy act, conscript her like a criminal and practically drag her kicking and screaming out of the castle, so I know that feel... she did grow into being a Warden and would have embraced the Commander thing like her life mission if she had lived to see it; but I've had other Wardens, like my Brosca and my Tabris, who didn't want anything to do with the Wardens even at the end of the game, and couldn't wait to dodge town the second Riordan wasn't looking.