After the Triwizard Tournament, the experience changes Harry, and then he rediscovers himself. Then he also takes up lordships, business and then marriage.
Draco fell inlove with Mal in year 4 but he confess to Mal in year 5 and after weeks trying to gain Mal's trust they began going out. After the battle everything changes, a new girl transfer, Astoria Greengrass. Draco started falling for her, after a while they endup together and they got married. Eventhough he regrets his decision but he can't turn back so he must lived with it. Draco left Mal for another girl , because there is always another girl.
( I add something in here that is not in the movies~~)