Tractors like horses should be mounted on the left side. Just like lawn mowers, the grass shoot is almost always on the right and you should never mount from the right. On most tractor all the controls are on the right and that makes it darn near impossible to get into positions without pulling hoses or levers.
I guess it was drilled into me when I was a about 10 and started mowing the yard. Dad saw me get on the lawn tractor from the right side. I ended up mowing for a little while standing up. Just my 2 cents.
Wedge