From posts #398 and 403:

#398:
"Sometimes the best way to fuck up your enemy is from the inside. Make them think you are stupid or weak. When their guard is down, you attack because you are the least expected to. Somestimes you might have to endure pain to achieve your goal."

#403:
"Note that this is also basically Littlefinger's MO throughout the series. He feigned weakness or service or loyalty (whatever the other party wants to hear) in order to put himself into position to eventually capitalize."

Littlefinger's long con from the very beginning was manipulating Lysa from the create strife between the Starks and the Lannister. He knew where and when he had to sacrifice for his long term goals (incidentally, the exact opposite of Cersei's MO):

- Put himself through a false relationship with Lysa, eventually marrying her, despite loving Catelyn
- Feigns alliance with Ned, Catelyn's husband
- Pushes Robin Arryn to lead the Vale, instead of himself, despite the other lords skepticism of Robin
- Arranges Sansa's marriage with Ramsay despite his own advances towards her (first when she's no longer betrothed to Joffrey then again when he brings her to the Vale)

From less than humble beginnings, he's moved from Master of Coin to Lord of Harrenhal to (basically) Lord of the Vale. Littlefinger is the master of manipulation, and he knows what needs to be done to get what he wants.

Sansa is learning from the best.