If I had to guess, a big part of Sebastian fanlove comes from the States. Most of us aren't as familiar with the accents used in the game, so if there are some really bad ones it generally flies over our heads. I think for me, my love of Sebastian came from his faith. He's adorable to torment and it was nice to see a character who didn't blow the Chantry off. You can make an argument with Leliana in Origins, I think, but I personally feel they did a better job with showing the Chantry with Sebastian than with Leliana.

And did I mention the torment part? Seriously, I have a brain like Isabela's and all of her banter with him was things I would have thought to say. Plus I think there's something to be said for fangirls and dangling a not really romanceable character in front of them. I agree with your thoughts on Varric, I just can't see him and Hawke together, but I fell for Sebastian/Hawke, though it's definitely not my favorite LI.