As my grandmother used to say:
"Your son is your son til he gets him a wife,But your daughter's your daughter all of her life."
It's exactly this attitude that I think sets the relationship off wrong.
It seems like some women consider it inevitable that their sons will behave this way, and more worryingly enough wives/partners who are all too happy for their parents to be the favoured family throughout.
Nobody seriously thinks everyone should pile round a new mum's house within days of giving birth, but the idea of one family routinely getting preferential treatment is evident in so many threads on here.
For example, there's a fairly obvious difference between:
A) woman has her mum with her as a birthing partner, when mum, dad and baby go home, woman's mum also comes home with the family and helps support her daughter in the early days. Woman says she wants to see visitors based on how she's feeling after giving birth and over the next few weeks family on both sides meet the little one and get to see their children.
And
B) Woman has decides that her family will be coming into hospital to see her and the baby and they'll be staying for a few weeks, but her partner's family can't meet the baby until the baby is a month old. At 8 months pregnant they've already decided this is how it's going to be. Everyone is expected to fall in line whilst the woman has her family round to stay at the house (regardless of whether DP wants to have house guests), man's family still ask when they can come round but they're told no, woman probably starts an AIBU about the terrible MiL.
I've seen it happen a few times where the focus seems to be 'new baby, mum and mum's family come first' and the dad and his family are expected to slot in behind. In at least a couple of situations it set the tone for the relationship going forward and it felt like there were 3 parents in the marriage: wife, husband and wife's family (who seemed to be privvy to almost every element of parenting and every discussion). Those marriages ended and it was easy to see why.