Just a little tidbit I wrote during class, figured it'd be worth posting. It's been a while since I've seen the episodes in question, and this was only proofread by me, so apologies if I get any details wrong.
Enjoy!

He had never been a bad kid.

Good grades, good friends, good family life. He'd bought the paint for a school project his classmate was working on-they'd share the work and share the grade. It seemed like a good idea.

He went, painted, and they finished with a little less than half a can left.

It was on his way home from his classmate's house that he did what was, perhaps, the worst thing he'd done in years. He couldn't say what made him do it. Why the blue box caught his eye, even if it never did strike him as out of the ordinary. Why his hand drifted down, wrapping around the can of paint still in his pocket, and pulled it free. His feet moved of their own accord, shoes thapping the pavement, no-one to hear them but himself.

He emptied the can, spraying the pale letters over the blue wood more naturally than he thought he'd ever done anything. Words formed from the letters, and then a phrase, and then he was done.

And then he realized he'd just committed vandalism, and he did the only thing a kid knows to do in a situation like that: he ran. He pelted home as fast as he could, discarding the not-quite empty can somewhere on his way.

He dashed in the door, stopping only to alert his mother he wasn't hungry and yes, his project was finished and no, he had no homework tonight, and then didn't leave his room for the rest of the day.

Two days later, when a man in a leather coat pulled him off the street to wash off the words, staring at him with eyes older than he thought he'd ever seen, he didn't even bother apologizing. He really wasn't sorry, even if he couldn't say why.

Besides, he didn't think the man would want to hear it anyway.

It wasn't until much later when the desire appeared again, in that same lot he'd sprayed only weeks before, this time with a bag of paint he'd been about to use for yet another project with that same classmate, that he realized it must be important, if the universe called him to do it so.

So he used all of it, spraying the message to the stars over as much concrete as he could get it to spread over, and it was only when he had exhausted his supply of paint that he left it be, discarding the empty cans in the first trash bin he passed, and leaving the scrawling words where they lie.

Bag on one shoulder, he set off. It was his friend's turn to get the paint anyway.