To be a little serious, They were tricked into doing something horrible, yes, but what they do normally is horrible. I sincerely doubt the majority of Thedas wants to know that they need to use crap tons of Blood Magic, separate themselves from their loved ones and self Ghoul-ification in order to fight a tainted dragon that requires you to die in order to slay it.
How many people would volunteer knowing what The Joining is?
"Hey bruh, we need you to ingest this concoction of demon dragon blood, darkspawn blood, and apple juice...the apple juice makes it go down easier. It might kill you outright but if it doesn't don't worry: it'll kill you between ten to thirty years. Side effects include gradual loss of sanity, ghoul-ification, and the inability to sleep comfortably due to the constant screaming of an angry darkspawn dragon monstrosity that may or may not have been a god at one point...by the way it kills your sperm count. A lot. "
and then:
"Hey bruh? Now that you're a Grey Warden, you can't be near your family because you need to ready in case a Blight comes. So you gotta train a lot, recruit more people, and oh by the way? If there is an Archdemon, it can only be killed by someone like you because it moves from Darkspawn to Darkspawn because they have no souls, so...yeah...if you kill it? Your soul gets obliterated."
Also:
"Hey bruh, you've been with us for awhile...remember those dreams? They've come back huh? Welp! You're gonna become a Ghoul soon. So you can either wait for it to take over you so you can be a senseless killing machine or go down to the Deep Roads and kill a crap ton of Darkspawn until you die."
How many of you would sacrifice everything you love in order to save it?
The Grey Wardens separated themselves from everything they loved because they knew they had to dedicate themselves to fighting the Darkspawn threat, in a way no one else would or could.
Who else can unite the various peoples of Thedas? Ferelden? Tevinter? Orzammar? The Chantry? The Qun? Mabari? What do they have in common? Very few things, and even those differ...but they all have the Grey Wardens.