One of the things they really drilled into our heads in grad school was that a baby is full-term at 40 weeks. Not a 37, not at 39, but at forty weeks, and even though they won't have any more life-threatening issues at that point than they would at 40 weeks, they are different. They have higher rates of jaundice and feeding issues, among other things.
So, for example, to pull some numbers out of thin air? You can have a baby at 38w5d and they will come into the world happy. Practically meditating!
But as they approach their due date, they will slowly wake up and notice the world around them. And they won't like what they see. At first, they'll just demand to be held 24/7, but that's okay, because you're all into baby-wearing, and your first baby was probably REALLY fussy, and if the baby is happy when they're held, they're happier than your first baby! Maybe they'll insist on sleeping in your bed, but hey, sleep is sleep, so no biggie. Then maybe they'll get really fussy in the evenings, and you'll think, well. That's less than ideal. But it's okay! All babies are fussier in the evening.
Then they'll be a screaming, raw bundle of nerves in the evening, wanting to nurse every single second, but becoming enraged when their mouth fills with MILK as a result (and some dumbass will ask if you've tried a pacifier, like you recently suffered severe brain damage and couldn't come up with such a simple solution on your own). So you'll stop nursing, and they'll arch and writhe and root frantically. Songs, rocking, bouncing, walking- won't help.
Then? This behavior will progress to the whole day. And probably, your two year old isn't on board with every second being devoted to calming the baby. Probably, she'll feel all neglected. And maybe she'll watch lots and LOTS of tv, which is really great for her speech development.
Just, you know, to cite a hypothetical example.
And probably, if that happened, you would feel like it was really an unfair pile of bullshit, because you already went through this and it was your turn to have an easier baby this time. Maybe this baby would want to be held all the time, maybe this baby would be crabby in the evenings, maybe this baby would even cry every evening, but to scream all day? No. You already went through that and feel absolutely no shame whatsoever about thinking that it was someone else's turn. Probably, you'll also be acutely aware that people alternate between thinking you're exaggerating, or that you're doing something to the kid to make them that way, and that just pisses you off more.
And then, finally, an hour and a half after big sister is asleep, fussy baby v2.0 will finally fall into a fitful sleep, and you won't even care about any of that anymore, because you are going to die and be dead if you don't take a nap.