Nothing seems to bring me to such raw emotion than saying goodbye to a pet. I don't know if it is the enormity of having to make the life vs. death decision to euthanize ( was it too soon, did I let her suffer too long, etc.) or if it is the fact that they love so completely and unconditionally.
Maybe it is guilt..."Could I, would I, should I have acted sooner, noticed the symptoms sooner, shut the screen door tighter and so on."
I know first hand that guilt complicates grief in significant ways.