But back to my original question, why is it that Cullen ended up being such a widely requested companion for DA:I? I remember him well from DA2. He seemed like a genuine good guy who was capable of putting reason before his duties, which is why he told Meredith to stuff it at the end.
Putting his story arc aside for a moment, largely it's because he's a bit of a mystery. We've seen him across the course of 2 games and watched from the sidelines as his story arc progressed, he's a returning character like Sandal and Bodahn who have continued to appear in each DA installment... a reoccurring theme that brings back a little familiarity to each new game when everything else about it has changed drastically. Of course, while it feels like we know him well in actual fact we know next to nothing about him beyond what the game showed us. We don't know what his life was like before joining the Order, we don't know his family background, we don't really know his likes and dislikes, what his off-duty personality is like or even what his Surname is (if he even has one!) And that's what's interesting! From a creative standpoint so far we've been able to project, speculate and theorise pretty much anything we want about him which tends to capture people's imaginations, but from a purely character driven standpoint the fact that we know just enough about him to peak our interest, but not enough to know him "intimately" the way that we have with past companions is what continues to drive our curiosity to find out more about him.
In regards to his story arc, (I think I can speak for near enough every Cullen fan residing within this thread) it's simply a case of finding his story an intriguing one. In DA:O he starts off as a naive boy who actually feels bad for the mages (something you only get to see if you play the mage Origin), of course later when the Ferelden circle gets attacked he becomes angry and bitter after being tortured for weeks/months at the hand of blood mages and demons, which understandably sends him down a rocky path. To be honest, after those DA:O epilogues I don't think any of us really expected to run into him again, and if we did then we just presumed that he'd either be an escaped mad man, a balls to the wall hardlined Knight Commander, or a completely broken man... so to see him pop up in DA2 as the Knight Captain serving under Meredith was a bit of a surprise! What's even more surprising is that despite his initial seemingly hard-lined attitude he does start to question Meredith and the Order, he gradually begins to realise that the way Meredith ran things wasn't morally right, yet all the while you can see that he's still suffering the effects of what happened to him back in Ferelden and is fighting between his heart and his head in a constant internally struggle as he tries to come to terms with the situation. If you play a pro-templar Hawke towards the end of the game Cullen will actually try to save 3 mages that come to them for help, Meredith wants them dead but he states his case and despite the fact that they might be blood mages he wants to spare them. The fact that he does this shows a BIG turning point in his character growth considering he'd been tortured by blood mages and watched his Templar brothers and sistes die years before hand. Aaaaaand then there's the final battle where he fights by Hawke's side to slay his boss... it's a big deal!
Sooooo, yeah, we just find him a very interesting and complex man. It's not often that you get to watch a character (specificially a non-companion) grow across 2 (soon to be 3) games from the sidelines without having any real input into how they turn out.