The Sefat Emet writes:
It is written MISHLE 23:26: "My son, give me your heart, and let your eyes observe my ways". Because the one who guards his eyes and his heart, which are the two agents of sin, merits that the Holy One Blessed be He put His eyes and His good heart over him for Good, as it is written: "And My eyes and My heart was always there" and it is also written: "He will not remove his eyes from the Tzaddik", because through guarding the Brit and through guarding these two agents of sin, the person is on the level of Tzaddik, and measure for measure, the Tzaddik merits the constant supervision through the eyes of the Holy One Blessed be He.