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.
There is more to the saddling process than throwing on a saddle and cinching the girth. Correct saddling and grooming prior to riding is key to having an enjoyable ride. The article below discusses the proper way to groom and saddle your horse.