There's a saying: Training a horse with fear doesn't work because there will always be something he fears more than you.
That's Not the Real Reason
Forget about "something scarier" coming along that undermines your horse's fear of you coming down on him like a ton of bricks if he "misbehaves." Misbehaves is in quotes because horses behave; they don't misbehave. The concept of misbehavior is something we generate in our minds and bestow upon them. But, I digress...
This unknown scary thing isn't what's going to transform your super docile and compliant pony into a fire breathing beast. It's the FEAR itself.
There's Nothing to Fear...
Remember that old adage? There's nothing to fear but fear itself. When it comes to working with a horse who is fearful, those words are right on point.
A fearful horse is not being calm and compliant because nothing scarier has come into his world to set him off. He's obedient because he's afraid. He's afraid to make a mistake and suffer whatever consequences he has learned will occur.
That's Avoidance not Cooperation
If you're being a "good" boy or girl because you're trying to prevent physical or emotional damage or distress, you might be a smart pony...but you're not a happy one.
Continuous exposure to frightening events is very traumatic and can cause long-term psychological disorders like Post Traumatic Stress Disorder, health issues, ulcers, reactive and/or unpredictable behavior, aggression, depression, stereotypies, and poor welfare.
What's Really Happening
A horse who has learned to fear humans will learn to associate them with bad things, such as pain, confusion, exhaustion, hunger, etc. In the context of being handled or ridden, this horse will be nervous and worried because he anticipates that something bad will happen.
On a side note, if the physical and/or emotional distress goes on long enough, the horse may begin to feel generally insecure and anxious even when there aren't any people around and show physiologic effects and have a hard time adjusting to new situations, become difficult to handle, or withdrawn.
The Neurology
When a horse, or any creature with a brain for that matter, is chronically anxious, it's survival mechanism becomes over-developed and hyper-active. This horse may be well-behaved and even appear to be calm.
Then suddenly, as if out of nowhere, he explodes.
Did "something scarier" come along? Probably not. In fact, it's likely that it wasn't something that's scary at all. Even something super familiar could set him off...because he's already dealing with as much fear and anxiety as he can handle.
In his higher hyper-alert (fearful!) emotional state, the slightest trigger could be all it takes to push the horse beyond his ability to maintain his composure. He's like a rubber band stretched to it's limits - a very small amount of pressure will make it snap.
Fear Works...?
Why would anyone use pain, fear, intimidation, coercion, and the other tools that teach horses that they are powerless to defend themselves and makes them unable to feel safe?
Because it works. It works really well. I'd be really quick to figure out what I have to do to avoid pain or other uncomfortable experiences. However, fear-based training comes with a hefty price. There is serious emotional and physiological fallout, creating a significant welfare issue. It also can't be sustained forever. Something has got to give. More often than not, the horse becomes over reactive. A lot of people have been seriously injured working with horses who are living in a fear-state. There's nothing good about it for anyone.
Ditch the Fear
We can teach our horses very well and much better (faster and more effectively) using fear-free training methods. Having wonderful experiences keeps the fear response from being triggered, so you are working with a horse who wants to engage and participate in the experience.