The fact is, it is almost impossible to “reinforce fear.” Fear is highly aversive, and if anything, it works in reverse. I suppose, if you did it often enough, you could create an association between thunder and petting that would make your dog afraid of petting, but it is extremely unlikely to go the other way around.
It is true that you can make your dog more afraid than he already is, by doing something yourself that scares him, by forcing him into situations that scare him already or by being afraid yourself. Emotions are contagious, so if you want your dog to be afraid of thunder, then be afraid yourself! But you’re not going to make him more afraid of storms if you stroke his head and tell him it’s going to be okay.
