I totally see your point, and it's most likely spot on. He surely does indeed see it as a good thing, and probably doesn't want to change it. Yet, it's also harming him. We know about the effects. That can't be a comfortable prospect, especially as he gets past the age where everyone sort of feels invincible, especially since the whole world is being ripped apart and the Templars could not have stopped it despite their powers. I would not mind at all if he thought about his own future and well-being a little bit, or if he realized there is more than one way to serve the Maker and that duty can take more than one form.
Now that we're discussing it, it does seem fairly complicated. Maybe it would be best to make it solely about the issue of red lyrium and leave it at that. Hmmm. I think I may have just changed my mind. 
It's a really interesting thread to follow, because it's really hard to argue that Cullen should put his health first when there are demons pouring out of the sky and we've dragged him into the middle of it all again.
INQUISITOR: Just... just think of the future! You would have a full, healthy, sane life ahead of you!
CULLEN: Is that before or after the demons completely engulf all of Thedas?
INQUISITOR: Oh, don't be so smug.
I think they kind of have to touch on it, at least a little. It's not like with Anders and the Taint, nothing about Anders meant he had to bring that up, ever, with Hawke (which is a dickish thing to do, but, well, it's a carefully guarded secret so I get it). Cullen, however, would have to be taking lyrium on a regular basis. Maybe not once a week, but regularly to prevent withdrawal symptoms from kicking in. And an Inquisitor who romanced him would notice that. There has to be some kind of discussion. Whether that culminates into Cullen getting cured of his addiction or not, I don't know.