What is Bruce Wayne’s worst fear?
Wayne's phobia of bats is the most apparent. It stems from his childhood incident of falling down a well with bats flying up above him. Because the incident happens at such an impressionable age, it causes extreme fear and becomes impossible to get rid of.
Batman's apprehension over losing a loved one stems from his childhood. This dread is related to his fear of bats.
Bruce Wayne, better known as Batman has chiroptophobia, stemming from an incident involving bats in his youth, as shown in the movie Batman Begins, but not in the comics.
Eventually, Bruce decides to take something that once haunted him and turn this force against his enemies. In doing so he turned his greatest fear — bats — to work for him, eventually becoming a symbol for the protection of Gotham.
Not to go too far down the rabbit hole of pop psychology, but what Hulk and Banner are most afraid of is losing themselves to one another. Banner is afraid of what will happen if he Hulks out, his anger takes over, and he can't regain control.