In 2003 I posted this at my former Yahoo group:
I had always held two theories about the fascination for dirty soles. The first is that in admiring or paying homage to matter that has been casually trod under the woman’s feet, one shows reverence for her by showing reverence for the mere grit that has been ground into her shoe/boot sole, the one personal object that is lower (physically and conceptually) than the “lowliest” part of her body. This would be especially relevant to sole-licking; the idea of orally incorporating random material which was so worthless that the lady walked on it without thought is a most humble and humiliating form of submission to her supremacy, and represents the logical extreme of foot and shoe worship.
The second theory, more relevant to trampling per se, is that being walked on by a woman’s real-life shoes reduces the tramplee to the status of sidewalk or carpet. It is a way of deepening the sub’s humiliation, treating him as nothing more than street pavement to be walked on with at least a pretend form of carelessness. The same would be true of being trod under bare feet with soles dirtied from walking around outdoors.
Please feel free to add your own thoughts and experiences