I think that it's actually interesting, if we're to "analyze" his personality/interactions throughout the series.
DA:O Magi Origin - a shy, moderate guy, quite nervous. This gives some colour to his person.
DA:O Broken Tower - heavily shell-shocked, currently freaking out due to his bad experience. Whew... heavily colourful if you ask me.
DA:2 - cold, hard person. Likely still recovering from above event. He's distancing himself from everything to protect himself IMO, this might make his character seems bland. Then, he distance himself from "emotions" once again to focus on his job. If he's a colour he's definitely grey there, cold grey.
DA:I - a more warmer person than in DA2, maybe that's because he had finally started to believe what he's doing is actually serve for the better, that he's getting a second chance of everything he felt he had screwed up so far in his life. Doing his romance, spending time with him, helping him cope with his "condition" in either way gave much colour to his person.
So, that's to say, I'm not at all surprised to see Cullen as a gray/cold character in DA2. It really gives the magnificent pallet of his character, we've seen it in every colour now right?
Personally, I wouldn't call him cold and hard in DA2, but to each their own. I actually found his interactions over the six years with Hawke to be quite warm, and I headcanon them as friends.
In any case, he has definitely evolved quite a bit, even just over the course of DA2, IMO. These are among the many reasons I find him interesting.