What system are you on? If PC, are you using any mods? If so, what?
Sera doesn't like to share, so if you're even a little bit in a relationship with someone else, she might decide you're taken and that's that.
Also, choosing the choices to gain the most approval possible means you're not always choosing the romance choices, which means the game thinks you're not interested.
If she wants to travel more, then do that. After a while you should be able to talk to her about it again. If you're only starting the Winter Ball quest thing, you're not all that far in the game, so you mightn't be able to get super far in the romance (can't remember what you can do when).
If you're at max approval with a follower, it seems they stop gaining more approval when you do things they like (not sure about this, it's just what I've observed in my playthroughs). You can still lose it, however.
You can also try a repair of the game if all else fails. Pretty quick and easy to do.
OK, I now can't even read what I've written, so I'm gonna leave it there. 😛 Good luck.