Brandon Stark took a big, publicly talked about, almost fatal, humiliating shyt on Littlefinger when he sliced him up.
Ned Stark married the girl Littlefinger wanted for himself and went on to successfully and happily breed all those kids with her.
Littlefinger is obsessed with ending the families that did him wrong...
Both GRRM and the show go out of their way to associate Sansa with her mom's side of things, making it easy to think that Littlefinger's coming from a place of caring about Sansa because he cared about Cat. I don't think that any more after last night's episode, I think Sansa is the last known living Stark and Littlefinger is out to end her and the Stark name. If he marries her to Ramsay, she becomes Ramsay Bolton. If she gets pregnant, the baby is a Bolton. The Boltons are the new Starks. Or maybe the North says fukk it and takes out the Boltons, Littlefinger can just smooth talk the next family that steps up and act like he was trying to do best for "Ned Stark's little girl". Either way, he wins. If Ramsay thinks like Bronn and Sansa has an "accident", that's all the better. There is no way that Littlefinger is doing this so he can rule the 7 kingdoms with a Cat lookalike, he's been playing that slow burn revenge game. I think he'll end up doing something very similar in the books to what is happening in the show, not with Harry the Heir, but someone similar to a Ramsay.