I went through the same enormous transition when getting married and having kids. And of course, we both knew ahead of time--intellectually--how big a deal it was going to be.

But here's the interesting part: there are other, comparably enormous transitions later in life--and for some of them, we know almost nothing about them beforehand, even "intellectually". Sure, we know that the part of life where the body really breaks down before death is going to be a huge deal. But something more vague happens in one's 50s, roughly put, that's also huge but not foreseen. If I were to take a shot at describing it, it's when the chickens come home to roost. It's when the shit you've been avoiding in your life forces you to deal with it. It's a big deal.