I suggest a theory:
God hates me.
Justification of my theory:
The whiter I want to keep my new shoes is directionally proportionate to the chance that I'll step in random dog shit. The frustration factor is multiplied by the fact that I a) do not own a dog, and by all rights should not come into contact with said species fecal matter, and b) hate the smell of said matter more than anything in the world.
It's not like I don't watch where I put my feet - I do. I suspect that some almighty entity is playing tricks on me, and conjuring piles of the stuff to magically appear under my shoe.
If you look at it, it's the only reasonable answer.