This may be a feature, particularly for mothers. The pain of the sleep deprivation and challenges of the early days of motherhood are not stored strongly, as to not interfere with the desire for more children.
Socially this means less empathy toward current early days/months/years parents though.
This is somewhat true. It isn't as though parents don't understand and remember the pain completely though. It's not amnesia. There is also a very tribal mentality (from what I've seen) in mothers. It's quite a beautiful thing where it is at the same time an exclusive community (you can only be a part of it if you have given birth), at the same time everyone who is a mother is automatically welcomed.