BJ: Actually I think it's pretty plain that both Sally and Irene were happy until Bernice started drinking & confessed her feelings to Sally when Irene was six. Irene had a breakthrough toward the end of the book and realized that the reason she couldn't remember anything before the fire was that she had been happy before that time, and hadn't truly been happy since. George also says that his mother changed when he was about six and was no longer a happy person.
Speaking to the question - I've heard before that people who are afraid of heights are actually afraid of wanting to fall, not so much the fall itself. George's belief that Irene is trying to get the better of her fears and that standing on bridges was actually a GOOD thing could therefore make sense. But personally, if someone I knew were exhibiting that type of behavior, I'd be very worried about them.