Some Siriusly Obvious (or not) Harry Potter Headcanons

Ron Weasley and the Mirror of Erised

As we know, Dumbledore is a very powerful wizard. It's revealed in SS that he doesn't need an invisibility cloak to turn invisible and he saw Ron and Harry when they were looking in the Mirror of Erised. 

In the Mirror, Ron saw himself as Head Boy and he's holding the Quidditch cup. Dumbledore knows that Ron has five older brothers that he has to compete with making it hard for him to impress his parents and siblings.

In Ootp, Dumbledore makes Ron prefect instead of Harry, and he later says that he didn't make Harry prefect because he thought Harry had other things on his plate.

I think that Dumbledore made Ron prefect because he knew that Ron had to work really hard to make his parents proud of him, especially because Harry is like the Weasley's son and Ron has to compete with him a little bit, too. Because of this, Dumbledore awarded the prefect badge to Ron in hopes that part of his heart's greatest desire would come true.