How They're Trying to Help: Well-meaning people are trying to prepare you for what's ahead so you don't get taken by surprise and go into a week-long catatonic shock one day, I guess. I think there's also an element of "welcome to the club," where they want to let you know that there will be someone there to commiserate with during your future suffering, a fellow inmate who understands your unrequited yearnings for sleeping in or Michelin-starred dining. So they're trying to be nice in advance.
How It Does Not Help at All: First of all, half of these things are, like rumors of Jackie Chan's death, grossly exaggerated. At some point between your child's birth and 18th birthday, odds are that you will, at least once, find a babysitter so the two of you can have a "date night," if you know what I mean. Also, it's pretty normal for many babies to sleep 12 hours overnight at some point, and at varying ages well before 18 your child will be able to go to a restaurant with you, then later take care of himself while you go to a restaurant, and eventually work at a restaurant so he can buy you dinner at a better restaurant.
It only feels like this most of the time. s**t, wait, some of the time. Wow, it's pretty hard to take this advice!
Even when these deprivations do come true for you, it's much easier to cope with them one at a time as they crop up. You simply encounter the problem, you go, "Arrrrrgh, nnnngh, I didn't know I was going to have to deal with this, everything sucks, life sucks, I don't believe in God anymore," and then a week later you get used to it and you're like, "OK, I guess this is part of the routine now, whatever."