Whether you're human or animal, events or environments you're exposed to in life work a lot like a piggy bank. The individual experiences you have in those environments or during those events are the coins in the piggy bank. Each experience is going to have a different value, and some experiences will withdraw while others will deposit. Every time you have a good experience coins will be placed into the piggy bank, and every time you have a bad experience coins will be removed from the piggy bank. What's more is that negative experiences are worth more than the positive experiences, so if a positive experience is worth ten cents than a negative experience is going to be worth fifty cents. Every time you have a bad experience fifty cents will be immediately withdrawn from the piggy bank... and when the piggy bank runs dry, or worse has a negative balance, you'll have nothing left to withdraw, and you may even be left in serious debt.
So what does this have to do with horse training? Everything.
Every moment of every day your horse is learning what's good and what's bad. Every single time you expose your horse to something new, take your horse somewhere new, ride your horse, walk your horse, feed your horse, or even just groom your horse you are either subtracting or adding money to that piggy bank. Horses, just like humans, are continuously changing, adapting, learning. Every single situation is either a good or a bad learning experience, and there is no such thing as "neutral", because even neutral is positive in it's own way.
To express what I mean a little more clearly let's look at three different outcomes for the same training experience. A positive experience, a neutral experience, and a negative experiences. After, we will explain the piggy bank analogy a little further.
Let's say we have a horse that has never been worked in a round pen. For the first example the human knows this is a great opportunity to set the horse up for success and spends a lot of time rewarding very small efforts, allowing the horse to explore the round pen and take things slowly. The human even decides to use