Post natal depression arouses from the female realization that shes been tricked into replicating beta male genetics.
Via typical beta male strategys - time spent, familiarity, submission, acts of service etc - she was tricked into replicating weak male genes.

She didn't love that guy, but hes just always there and always trying - and she slipped up.
She hates herself for this mistake.

This is why women in "happy marriages" get post natal depression and women who have 3 week flings with sports stars never do.
Because although the star barely replies to her and he doesn't see her - at least she birthed a warrior.

Remember - 1% effort from a killer is worth 1000% effort from a man he could kill. Always will be.

Genetics rule all.