You marry Margaret, the future Robert II's daughter. This improves your position. Not only are you on good terms with David II and the English Crown, but also the family of Bruce.
Once again, you've secured a strong position through well-chosen alliances.
In 1371, David II is succeeded by Robert II. Robert's claim to the throne is through the female line. He arranges marriages for all his daughters to powerful Gaelic lords. To secure his bloodlines and his throne, he needs heirs with roots in the Gaelic community.
Times are changing. The monarchy and people of Scotland are coming together toward a more modern community of kinship. Divisions that previously ripped the country apart, with war and bloodshed, are being brought together. Who knows what the future holds?