Maybe it's because she's the outsider and doesn't fit in the same as the others? She's not there in any official capacity and you aren't contracted to pay her like a merc.
idk really but I almost selected that option by accident a few times xd
This is actually extremely aggravating to me. I freely admit, I wasn't sure how I felt about Sera at first, but I'm at the end of my first playthrough, and I really like her now, and having the game ask me to make SURE I want her around every time I speak to her, when it isn't that way with other companions, feels like it's a statement about how we as players should feel about her.
I mean, you can kill Zevran without ever talking to him, but his dialogue options didn't even give me this kind of impression, that he was meant to be chased away. MEH