I couldn't really blame anyone for having negative feelings about spirits. Their presence in the physical world is usually accompanied by bad sh*t happening. It's easy for Solas to be all nice about it, seeing as how he's pretty much a supernatural entity himself, but for normal folks, it's basically scary ghosts that screw with people, possess minds (or extract thoughts against your will), and turn mages into destructive monsters.
Yeaaaah... And yet, we discover the Avvar, a culture that is 200% totally ok with spirits and don't fear them at all. My Lavellan ain't scared of them, and she loves Cole like a little brother. Sera doesn't even try, and that's what bothers me. She doesn't try to understand a lot of things that goes beyond her limited world views and opinions on the world. As soon as it challenges them a tiny bit, she shuts down like a little stubborn baby, and I hate that about her.
Just the fact that, when I romanced her in my first playthrough, she demanded that my Lavellan made a choice between staying in a relationship with her, and admitting that maybe elven culture is interesting and deserves some questioning, made me wanna slap her in the face. Such a narrow-minded girl. I understand why she is how she is, and I understand that she sometimes tries to make an effort, but having your partner make you chose between her and her questioning of faith and thirst for understanding, was a really huge dick move. I stayed because I didn't want my Lavellan to end the game alone, but had it been a real life situation, I would have gotten the fuck out of this unhealthy relationship.
I love her, but not when it comes to her prejudice towards things she doesn't make the slightest effort to try to understand. I hate that kind of mindset in people, so of course, despite all her other traits I enjoy, I'll always hate that part of her and wish she could outgrow it one day.