One of the first words that I remember the Bubbers (Brian and Mindy's twins) saying is "shiiz" in a really excited voice. They would always be holding up at least one of their shoes with a big smile on their face and it was there way of letting you know that they wanted you to put their shoes on.
Well, it must be a Burns thing because Charlie also loves shoes. If he ever sees his shoes, he rushes over, picks them up, and then walks over to you with a big grin while waving them in the air (and usually making his "ah-ah" sound (which means he wants something) at least once or twice).
Jenni says that he just "really likes shoes", but my theory is that, like the Bubbers, he associates shoes to playing outside and going to the park and that's why he always wants to put them on. Who know what the real reason is and I guess it doesn't matter, but one thing I'm sure of is that the little guy sure does love shoes.