To be honest, I was really puzzled when I found out some people hated him; I thought Bioware made it fairly clear that he was a wise, kind and compassionate character. I guess these qualities don't mean anything to the people who are happy to just react negatively to his personality (which can come off as a little patronizing if he doesn't like you) and not give it a second thought. And I think the problem is that it's just too easy for these people to say "because I just hate him". (There was a thread on the DA subreddit yesterday and you can see those responses there).
People really hate elves in the DAverse (sucks to be them when ELVES RECLAIM THEDAS. GLORY TO THE ELVEN EMPIRE)
Ahem..
So, I can see how he is an easy target.
He is very serious, he doesn't smile much, he can be a tad hypocritical in some of his party banter, he's pro-mage (which is another unpopular opinion), he doesn't like the Grey Wardens (which a lot of people are biased towards because of their love of their HoFs), he isn't happy at whichever option you pick at that damn Well
and of course, if you don't find him an appealing character, the truth about how Cory got his orb isn't going to make you suddenly like the guy.
So I can see why people don't like him. But they may also have not bothered to get to know him properly either.
I personally like all the DAI companions now. Didn't in the beginning, but I've run with them all and made Inquisitors that fit well with different people so I can see them with a different mindset.