Nature gave your horse one answer to pressure: leave. That answer kept horses alive for millions of years, on open plains, in a herd.
Then came stables, trailers, clippers, shows, traffic, a rider on board. Nature never provided the tools to feel comfortable with any of it. So when tension rises, your horse does the only thing nature taught, and we call it spooking, bolting or being difficult. Your horse isn't being difficult. Your horse was never taught another way.
Your horse isn't reacting to the object, but to the tension in his own body, and nobody ever showed him what to do with it. In controlled, small amounts, your horse learns to notice that tension and find the way back to relaxation: first on the ground, then with the tools, then under saddle. Not braver because he stopped caring, but confident because he knows what to do.
That is why riders so often see something shift in the very first session: it is the first time the horse is asked the right question.