Why? All I can see is that he didn't go through the fire yet, hasn't experienced the kind of adverse circumstances Calpernia went through, but that's not his fault.
To start off with, I do not think much of Dorian's decision to flee Tevinter when he had the status in its society to rise to power and work to bring some changes. If he did not care about his homeland and just wanted a happier life for himself, running away would be okay, but since he claims to want things to change there for the better, I think he threw away an opportunity.
But never mind that.
Where Dorian really lost some points with me is how he reacts to the Inquisitor if he/she made Alexius tranquil. I never did that, myself, but I saw it on YouTube and it kind of disappointed me about him. He greatly disapproves, but does not offer much to contest your decision other than "I wish you hadn't," and that just made him come across to me as really, really weak -- a great injustice is committed against a man whom he had great respect for and his reaction was as potent as a passing sigh. Is this the guy whom we believe will bring positive change to a nation that is corrupt to high heaven? Even Alistair had more spine than that.
I like Dorian well enough. At this time, though, I do not see him going on to do great things. That will require a fire that I did not see in him at any point in Inquisition.