I think that Proctor's mood change is due to a couple of elements. The first is that Hale's speech brings out how the accusations and the imprisonment that follows is reaching a feverish pitch. Hale intimates that it is out of control, and Proctor begins to understand this. In his own mind, Proctor is also slowly moving from an isolationist point of view to something more active. Prior to this point, Proctor simply did not want to get involved. Proctor wished to remain distant. Hale's speech ends up bringing some intense doubt to this proposition. Simply put, Proctor recognizes that he must do something and that his desire to remain distant is not going to be feasible. At the same time, Proctor is also beginning to realize that Abigail is a force of malevolence that must be stopped. Contrast this to his earlier position on the issue at the start of the act, and one can see how the change is evident. Proctor also realizes that the inquisition he received at Hale's hands about his own spirituality is something that will become an issue at a later point. These elements combine to move Proctor to a new point of view, one that will set the stage for his change and evolution in the course of the drama.