[Saurreg]

Perhaps Denethor disliked Gandalf because he secretly feared the grey pilgrim also. This fear could stem out of sensing an aura of power and wisdom around the wizard and also the lack of knowledge of Gandalf's origins. Denethor being a proud and masterful man must have wanted to have a grasp on everything and he would not have felt secured with the mysterous presence of one person who held sway on so many, much less his own son.