The Bear and the Maiden Fair (307)

[Skydiver]

While many characters have ulterior motives, not all of them do.
Margery, Cersei, Jamie, Joff, Tyrion, Tywin....they all have their reasons for doing things.

but there are some that only have one fish to fry.

[Selina]

I was wondering the same thing, when I saw her writing that letter, because:

Spoiler:

In the books it is hinted at that the family of Robbs wife (but probably not herself) might have been involved in his downfall.

But the way she acts and everything she does and says really seems sincere so I don't think that the is a spy/saboteur whatever.

[Skydiver]

yeah,

Spoiler:

in the book her family is most likely at least conspiring, but on the show she's totally devoted to Robb. so one more thing that's changed