I think he really did, honestly. Of course, the details of the job weren’t always great (very, very few people really like fighting darkspawn for a living, and the Deep Roads are unsettling enough for those who aren’t claustrophobic and afraid of the dark; I can only imagine how horrible they’d be for Anders), but I think what he got out of it was far more important than that.
Because assuming the Warden-Commander has a good relationship with him (my main method for playing Awakening is basically ‘start by burying Anders in gifts until he can’t move, then get to the story bits’ lmao), it actually was a very pleasant experience from a social standpoint. His commander liked him, gave him all sorts of nice things (including Pounce, his best friend/therapy animal), he had friends, maybe a lover (depending on your headcanon), acceptance, and no one was going to force him into a cage any time soon. From Anders’ point of view, that’s basically the jackpot.
I mean, as he says, the Warden decides that he’s “worth the trouble” despite him being “usually a lot of trouble”, which is something he’s not really used to. I personally believe that by the end of his time in the Circle, he had few friends left due to just how much trouble he got into. And of course, he spent most of his later years escaping or enduring punishment for said escapes. Doesn’t exactly win you many friends.
But then he ends up with the Wardens through some twisted turn of fate. The Warden-Commander treats him well, likes him, and actually helps him with destroying his phylactery (even though it was a trap). They defend him, and continue treating him well despite that making things difficult for them. And as more people are recruited, Anders finds friends. I’d argue he got along better with the Awakening crowd than the Kirkwall crew, and these people probably did start to feel like a little bit of a family. He found something in Amaranthine he’d never had before: acceptance. These people liked him, would fight with and for him, and weren’t turned off by him being a mage or his delinquent ways.
So yeah, I think he really did enjoy it, and that he truly did mean to stay for life until everything went to hell. It seems basically that after the Warden left, that’s when things really got bad. He lost his family, then his cat (which I think was the last straw, tbh. After Pounce was gone, he no longer had ties left, so that’s when I think he decided to leave and fight for the rights of mages, merging with Justice in the process). Then he merged with Justice, and was attacked by Templars despite the fact that he was supposed to be safe from them here. So all of that bad piled up and he didn’t see any other choice. He felt trapped and alone, something he very much doesn’t like, and that’s why he left.
Had the Warden-Commander been allowed to stick around (because Bioware basically said ‘fuck player agency’ with a lot of what happened there), or if Anders had gone with them, or if the new Warden-Commander hadn’t been a dick, I think he absolutely would’ve stayed for life. Everything he’d always wanted was there. If you think back to how Alistair describes what being a Warden was like before Ostagar, that’s basically what Anders had. Despite the darkspawn bit, they had fun. They were a family who loved and accepted each other, despite most of them being grumpy children. They were bonded through blood, just in a different way.
And if he hadn’t had all of it taken away, I don’t think he ever would’ve left. Because he was happy. And, thanks to the World of Thedas vol 2, we know that when Anders is happy, he stays put. It’s not until things/people/creatures he loves are taken away that he tries to run, and that’s exactly what happened post-Awakening.