2 Samuel 3:13

And he, David, said, Well; I will make a league with thee; but one thing I require of thee, this was his one condition upon whose fulfillment he insisted, that is, Thou shalt not see my face except thou first bring Michal, Saul's daughter, when thou comest to see my face. It was not merely that Michal was his lawful wife, but David had also a political reason in his union with Israel's princess, namely, that of gaining the favor of the northern tribes.