From his first introduction into the story, Kenny regards Rufus as his "personal saver," someone who will stop him being bullied because he was a figure that didn't fit in even more than himself. However, he is shocked in Chapter 3 when Rufus enters Kenny's class and has to sit next to him:
I couldn't believe it! I'd wanted my personal saver to be as far away from me as he could get. I knew when you had two people who were going to get teased a lot and they were close together people didn't choose one of them to tease, they picked on both of them, and instead of picking on them the normal amount they picked on them twice as much.
Thus Kenny is selfishly pleased to have Rufus in the same school as he thinks this will reduce the amount of bullying he receives, however his plans are in danger of going awry when Rufus is sat next to Kenny and thus the two are linked, ensuring that they will together be bullied more than Kenny was ever bullied by himself. This is why Kenny does not want to spend any time with Rufus.