He didn’t like McGarrett initially.
Six months after arriving in Hawaii, Danny was assigned to the murder investigation of Steve's father, John McGarrett. Steve had used his influence to become the leader of the unnamed task force working on the case, and appointed Danny as his second-in-command—and partner in the field. Unhappy with the situation, Danny got into an altercation with Steve. Despite the rocky start, they've since made up and have become very close.