I'll drop a wall of text and then stroll off. Like I always do.
And this is the spoiler forum. So I won't use spoiler tags.
Guess I'm one of the few people who actually liked Solas before the game release? (I loved Dorian too, by the way. I was hyped for them both). I found Solas fascinating, and I still do, even when I'm playing a character he's mad at.
I recruited the templars as dwarf, and it took a lot for me to do that because it goes against what I believe to be right within this fictional world. But I did it. For science. Solas didn't agree with what I did. My dwarf told him "beggars can't be choosers" and he conceded to her point. He was pretty decent about it.
And he even managed to make me feel guilty because he's an apostate (who has already been threatened once by Cassandra) and now I've surrounded him with templars. But I head canoned that my dwarf would have protected him and told him so. After all, they were still friends.
It's take TWO major decisions to really p-off Solas. My dwarf will exile the Grey Wardens (for reasons I won't go into because off topic) so he will remain her friend.
It does suck the way he'll tell you "your people are crap . . . except for you." We have to remember that he comes from a time when no other races existed but for elves (we learned this in The Masked Empire, but this should have been made obvious in the game, which sucks). By default, all races are inferior to him until he is convinced otherwise. He literally has a god's mentality because . . . he's a god.
He also has not been in the world as long as Mythal, who has witnessed the races firsthand and knows they aren't all bad. Solas is still learning. And given that he wants the foci to do something that will put the world back the way it was, it probably comforts him to think that people aren't worth saving. It will make whatever he's going to do much easier -- something he will say to a Dalish IQ he hates.
Solas may be there for his own agenda, but he sincerely wants Corypheus shut down. At least that puts him one up on Vivienne, who is only there for a power grab.
tl;dr: Solas is an ancient elven god who values intelligence and free will, not blind service to the Chantry. That's why.