Pets provide so much encouragement, love, and comfort that their loss can be a huge blow-especially when it's unexpected or brings with it an element of guilt. Tack this debilitating event onto the end of a string of minor conflicts, and it can be the proverbial final straw that pushes a character over the edge.

Needing to do something with all the pet products in the house because looking at them causes too much pain
Having to make final arrangements (if the pet is going to be cremated, preserved through taxidermy, etc.)
Having to explain the death after the fact (when the vet's office calls months later with a reminder about a scheduled appointment, to neighbors who ask about its absence, etc.)
Seeing pets that look similar to one's own, and being reminded of him or her
Having to get used to new routines that don't involve the pet (going for walks alone, riding in the car alone, sleeping by oneself, etc.)