Why does the narrator compare the dirty pair of socks to a "sad little salted
slug"?

Reason for comparing the dirty pair of socks to a "sad little salted

slug":

A slug is a small but slimy creature that gets dehydrated when enough salt is poured upon it. It loses water, shrivels up looking dirty and eventually dies.  Apparently, a salted slug is not a happy creature as it is sure to die as soon as it loses all its water. Thus, it is a ‘sad little salted slug’.

The narrator in the three minute fiction ‘Not That I Care’ describes one of the pair of socks left by a character Marcus making use of simile, a literary device of comparison. The comparison is made between common characteristics of the two things compared.

Out of the two socks forming one pair, one lies in front, easily visible, dirty and all curled up while the other one is hiding under the bed.

The comparison implies that it was mucky and repelling like a salted slug.  It was lying unattended like a salted slug devoid of vitality. Simply thrown away carelessly in the house, it seemed sad too. The narrator had no intention of touching it, so the chances of its revival were bleak.