First, She's Engaged to Joffrey
Sansa's not totally blameless for her engagement to Joffrey — she's totally smitten with him when the young Prince first comes to Winterfell and ecstatic when their union is approved. But Joffrey's true colors come out immediately, and by the time her father's head is on a spike (and she's forced to look at it), she knows what a terrible, terrible mistake she has made. Luckily, Margaery is more ambitious than she and graciously takes the evil young king off her hands. If only that was the end of Sansa's troubles.