There are a couple reasons that a horse might not want you on his back. The horse may be afraid of being ridden or maybe he just doesn't like to be. In order to figure out how to mount a horse without problems or danger, you need to be able to determine if the horse is responding from fear or nervousness or if he just has a negative opinion of the process.
Fear is probably the least talked about issue with horseback riders.Somehow people are made to feel inadequate or stupid if they have a fear of riding, and yet fear is a very important emotion that deserves respect. Fear keeps you safe; it stops you from doing things that may harm you.