Because a stubbed toe represents the banality of evil in everyday life.

You can be a) happily going about your business, ambling along peacefully thinking about all that's good in the world - then BAM! you stub your toe, and it brings reality crashing back to you. And that sucks. You swear through gritted teeth. And your good mood? Gone.

Or b) you are already stressed, in a hurry, running to wherever it is you have to go, you think you're going to make it... then you stub your toe. And you're forced to stop, hop on the spot for a few seconds, losing valuable time, and carry on... limping. Oh, the shame.