I agree with Liz, Trillian, Lulu and some posters in between!
I think there is an element, also, of "my life doesn't have to change now that I have kids", in that parents seem increasingly unable or unwilling to 'give it up' for their children.
I think it's a hell of a lot easier to parent wholeheartedly and being absolutely child-centred when you're at peace with your new and changed role in life and that is means you are no longer your own person...for the better, because you're now putting yourself into a whole new person, a person who could well be a far better version of you!
The term HM=HB or even the converse suggests a dichotomy, or even a fission between the two entities. It's like in birth and people say it doesn't matter how the baby gets here, as long as baby and mummy are healthy and alive. It has become too easy to separate baby and mummy into two separate entities (and that's how it's easier for medicos to separate babies from mummies for hours on end before the first BF and natural bonding can occur etc), and this is what I believe leads to the ability to even think "Happy Mummy = Happy Baby", in the sense that Mummy's needs must be met first before the baby's needs are considered to be as vital.
We don't give enough confidence to new mothers to believe that they contain everything their baby needs. We advertise that they need all these mummy-substitutes like fancy buggies, swings, bouncers that go 'ping', playpens, whirring toys...everything except what the baby actually needs and wants...and that's a mummy who is existing for this timeframe to provide suvival and developmental stimulation just by being present with feeding, touching, communicating.
I think that the more mummies disconnect from these things and the way nature designed us to parent - carry, to co-sleep (cos other mammals sleep together!), to breastfeed (where physically possible, cos throughout the mammalian world you see where BFing doesn't work out, and it's not because of worrying that milk supply is diminishing or that it hurts too much), to PLAY with and teach survival skills - the less mummies are able to see themselves as interdependent with their babies and will see babies as somehow burdensome and stressful.
For me, once DS came along I was able to shift to a whole new consciousness - we were ONE entity and it really was a chicken or egg sicho with our happiness - one equals the other at exactly the same time as the reverse was true. It was not possible (and still sort of isn't) to suggest to me that by doing something that made me happy that might not be relevant to DS I would then have a happier baby. It made no sense and still doesn't. No, I would not feel better expressing to have a night out on my own. My place was with my baby because this time really is so short and how many people, during my pregnancy and after DS was born, were ready to tell me that the time goes so fast, make the most of it? So I did exactly that. Even during a horrendous initial Tongue Tie issue that directly impacted on breastfeeding, I made sure I was in the moment and didn't wish the time away, because that would wish away all the other magical moments that you just can't replace 'when you're ready'.
Anyway, all of this is quite aside from PND. Women who don't have PND who believe HM=HB and tell themselves this to justify their decisions that affect what the baby needs and wants are most likely not benefiting from people in their lives who support them in their new role and have caregivers who will happily brush her off with a HM=HB line so that they can tick her off their list and ensure their clinic gets government funding for meeting their quota.
It's just no longer about 'you' when the child comes along. There is another person whose needs are actually probably more important because they can't meet their own needs and need their primary caregiver to recognise and act on this.
There, I'm having my own rant, Lulu! Had to spread love, sorry chick