Why does toohey want keating to marry Dominique in the fountainhead?

Toohey wants Keating to marry Dominique in order to control and manipulate their relationship for his own benefit. By marrying Dominique, he can exert power over Keating and use Dominique’s influence to further his own agenda. Toohey is a master manipulator who seeks to exert his influence over others in pursuit of his own objectives.