Aladder is formed by rungs that are perpendicular to the sides of the ladder. explain why the rungs of the ladder are parallel.

The rungs are all perpendicular to the same sides of a ladder, so they are presumably oriented the same and are parallel to each other. 
I will point out that rungs can be perpendicular to a shaft without being parallel. For example if the rungs formed a helical pattern like DNA, they would still be perpendicular to the long axis, but not parallel to each other. 
For most ladders, the rungs will be parallel.