Horses are prey animals. What this means is they are always on the lookout for predators in the their environment. They are highly evolved in this way. If you move a bucket from one place to another, they will notice it and immediately have to decide if this object is a predator. They are not thinking it is a bucket that was moved, they only know that it was not there before so this is cause for alarm. Once they are allowed to see that it is harmless they feel safe with it again. They have to trust their handler for them to be brave enough to explore these scary things. The more trust they have the easier it is for them to recognize the object and move on. The same thing applies under saddle. The more trust they have in the rider the easier it is to get them through those obstacles that may cause them alarm.
The horse is a herd animal. In this social structure, there is always a leader, an alpha in charge. That alpha is responsible for keeping the herd safe. As long as the horse considers you, their rider, handler, trainer etc as their leader, you can build a strong healthy relationship built on trust and the horse will do just about anything for you. If this connection is not established right away then you end up with horses that bully their way around people. Horses must be treated like horses. They are happiest when they know who is in charge of the herd.
If you think like a horse you will understand why they respond or react a certain way and know what to do to help them understand your request. They are governed by fight or flight instincts. If they are in pain, they will fight to avoid the pain. If they are afraid, they will run away from the fear. If they emotionally cannot handle or understand what you are asking of them they will shut down.
If you are having a training issue, check to make sure you are being clear with your aids, the saddle fits properly and your horse is not in any pain. It is your responsibility to be precise with your request. This helps the horse to remain calm and want to perform for you. Keep your emotions out of your riding. Your horse will feel your tension and respond accordingly. Learn to ride well so you are not banging on your horse's back or hanging onto the reins. Think like a horse when you ride. This will help you to read what your horse may be thinking of before they have time to react in a negative way.