Riders almost always mount from the left side. This drags down the left shoulder and thoracic vertebrae, which will cause the muscles to weaken there. It also seems that there is some machismo involved with being flexible enough to mount from the ground. While impressive, this is not doing your horse any good.
Try to alternate which side you mount on daily. Your horse and his back will stay much healthier. If your mounting block is tall enough, you don’t even need to use a stirrup. Just swing a leg over and drop softly into the saddle. I remember working at a former Olympian’s barn in the 90’s and the mounting block was so tall (to mount the huge warmbloods boarded there) that it was virtually like a staircase. Mounting never involved pulling down on the saddle. You mounted from above.
Use one like this to save your horse’s back:
So much better on the horse’s back: