You've also got to allow for the fact that, reading between the lines, there are more than one set of parents at the house?
When my DS was younger and I used to go to the parent and baby/toddler clubs you'd hear people comparing sleep, toilet training, cost of clothes, lack of 'me' time. It's empathising, it's comparing, it's figuring out that everyone feels like this sometimes or all the time. It's checking that it's 'not just you' who is going through this, that you're not a bad mum or dad for feeling like that.
Now, when I get together with parents of children similar in age to my DS it's often a conversation about schools, hobbies, bullies, how much TV/gadget time it's 'right' to give a child, how to deal with the pre-teenage tantrums, how to keep them safe from harm when they become independent. To people who don't have children or whose kids are before or after that stage it looks like we're moaning. We're not. We're verbally back patting - 'we know, we've done that, we've seen that, we've been through that'.
I'm guessing it'll be what job/college/university conversations next, along with the serious girlfriend/boyfriend staying over, weddings, babies, spoiling the grandchildren and getting along or not with your DC's in laws!
The concerns, the comparisons, the 'am I thinking/doing the right thing' never stops. My mum worries about me getting home safe if I visit her on a wet or dark day - she's in a care home and I'm in my fifties! She's my mum and she'll worry about my life until the day she dies. That's the difference between being a parent and being paid to care for children. Even though you worked 60 hours a week I doubt you laid awake at night worrying that the DC was being bullied, is bullying, looking tired, didn't have friends, has the wrong friends, wet the bed, is eating too much, not eating enough or all the millions of things that keep parents awake at night sometimes. That continues through the child's life.
Your partner's family are just in a rut of conversation. It's like some brides to be get fixated on the wedding or people who are planning a big holiday can't stop talking about it.
My friends who, like me, have family with dementia, talk about the hardship of that over a coffee. We're not expecting answers, we just want to rant about the unfairness of it all.
And why do 'these people' decide to have second or third children? Because sometimes the first child is an angel to care for and the next is harder. Because they enjoy the early years. Because they could afford two, three or four children until x lost their job this year. Because they want their child to have a sibling. Because they had an 'accident' and became pregnant again. Because they love kids (but hate the hard work some times).