One of the most frequent phrases I hear starts with "I don't understand why my horse....." The very simplest answer of all is because he is a horse or because he can! For example, if someone asks the question "Why does my horse bite?", the answer IS because he's a horse and because he can. The deeper reasons are that he might feel the need to protect himself, to control his life, to make love to something, to confront something, to provoke a response such as play. Those are all good reasons why a horse might bite. One pony I go to see is terrified, absolutely terrified, of humans going near her and she copes with a six weekly farrier visit but shakes throughout and goes into pressure when he picks up her feet. Her owners ask me why, after all their years of kindness is she like that? They know that in the past she was mercilessly beaten. She can't forget that. If a human had gone through that experience, they wouldn't forget it either and they might have had years of counselling and rationalising what happened. Put them in the same situation, recreate the exact environment and it would trigger those memories, the same adrenalin rush and the fear that they were going to be beaten to death. Nevertheless, the owners ask me the same question every time I go. In their case I think they are more lamenting the fact that she can't let it go and it just fills them with great sorrow that after all this time she still finds it so hard to trust humans.
In other cases, where I here the same phrase repeated even though I have given the most logical explanation for the horses behaviour from the horse's point of view, I begin to wonder whether the phrase "I don't understand..." really means "I don't accept that my horse has a good reason for..." because I know that the words I use are in plain English. Sometimes it is hard to accept an inconvenient truth, such as the reason why your horse is so active around you is because you're his only form of entertainment because he has is bored, or the reason your horse keeps nudging at you is because you give him intermittent hand treats from your store in your pocket. So much of my job is about addressing the reasons for the behaviour as well as interrupting the behaviour and redirecting it. Sometimes it would be unethical to take away the horse's opportunity to express himself because he has a good and valid reason for it.