I think the root of it is the "It takes a villiage..." thing. It not only takes a villiage to raise a child, it takes one to make 'em.
Seriously, people just have a natural curiousity about children. Mothers of babies--how often do you take your babies out and have complete strangers come up to you and talk to the baby or make comments? If you are alone that doesn't happen, right? I think humans just have a inborn natural curiousity about children, especially babies, and that causes them to interject in ways that aren't really any of their business.
I admit, I used to be one of those people. "Don't let them outnumber 'ya" I'd say. Of course, I only said this to people I knew REALLY well who would know I'm joking. In my mind--3 kids, 2 parents=10 dentist appointments a year. What a PITA.
I always got the comment, "You should have more" when I had two kids. That used to P me off. Then I got pregnant. Someone actually said that to me while I was pregnant but still not telling anyone. So I answered, "OK, I'll get right on that. Honey, are you ready?"
When I got pregnant with my third, most conversations went like this:
"Congratulations! Is this your first?"
"No, it's my third."
"Wow, how old are your other kids?"
"11 and 8"
"Oooooooooooooooooh"
My mother had me when she was 39. My brothers were 14 and 16 at the time. My conversation with her went about like this:
"Mom, I'm pregnant"
"Oh, was this a surprise?"
"Yep"
"Yeah, you were too."
That was the first I knew I was an OOOPS! I thought they had tried for years to have me! Oh, well.
I should add, having this baby (she's one now) is the MOST FREAKIN' FUN I'VE HAD IN MY LIFE.

And my big kids (now 12 and 9) ADORE their little sis.
ETA: CONGRATULATIONS!!!!
