If you were to do a Top 10 list of the things you love about Cullen, what would make that list?
1.) That he's a flawed person who makes serious mistakes, but in spite of that he constantly and sincerely tries to do the right thing.
2.) That in spite of being placed in a very stressful and confusing situation in Kirkwall, where it would have been so much easier to slip into being completely jaded, Cullen continues to care about people, from the templars under his command (as we see when we first meet him) to the mages in his charge (as we see in the templar ending.)
3.) That he was not able to heroically shrug off the trauma of being tortured, as fictional characters too often do, but instead he was realistically addled by it in the immediate wake of the events and also continues to suffer after-effects. That his suffering is not glossed over.
4.) That in spite of #3, Cullen seems to be making significant effort to overcome his trauma. By the end of DA2, he is able to articulate opinions about the events at Kinloch Hold and mages in general that are extraordinarily reasonable and forgiving under the circumstances. That doesn't just happen by chance - to me it suggests that Cullen has been actively processing these things for years and trying to come to terms with them, which takes a lot of strength of will. I admire that he has the courage to do that rather than just allowing himself to stagnate.
5.) That he is willing to put protecting others before his own life.
6.) That I imagine him as a man of deep but quiet faith.
7.) His combination of shyness and confidence - that he is clearly an introvert making an effort to be more outgoing for a job that requires it, which I think a lot of us can relate to in our own lives.
8.) His ability to remain reasonable in Kirkwall even when most people around him were descending into various forms of zealotry, which shows his strength of will and of principle.
9.) That if the female mage in DAO doesn't awkwardly sexually proposition him, he's so sweet when he offers to talk to her "any time."
10.) Great fan community, so there are wonderful people to discuss Cullen with. Very kind voice actor is icing on the cake.
How's that? 