Robb decision to marry you, a Targaryen, was a direct affront to the Lannisters and their supporters who had backed Joffrey's claim to the throne. Even some of Robb's followers weren't thrilled with the idea, given the atrocities committed by the Mad King against the North. However, Robb understood that these were not the crimes of the Mad King's children, wife, or any other Targ. He knew that they needed the dragons, the fire, and the strength of this alliance to win the war.
As a Targaryen, you were one of the last surviving members of your family after Robert's Rebellion, during which your entire family was executed by the Baratheons and their allies. Robb's plan was for you two to marry and create an alliance that would help the North defeat the Lannisters and the treacherous brat king who had falsely accused his father of treason and had him killed.