Why is my boyfriend terrified of thunderstorms all of a sudden?

We were just standing by the window watching the thunderstorm rolling through southern illinois and he started shaking. He always is terrified to come over to my house if its about to storm..i always thought he was just using it as an excuse but he is really scared of them...this just started after May 8th inland hurricane...but he didnt get hurt or anything, so why is he scared now??