Before I had a baby I don't really think I appreciated just how earthshatteringly life changing it is. 
Over a good nine months you share your body with another being, you get to know their habits (generally kicking and punching at random intervals), you feel you know them.
No one that hasn't been through it can understand the wonder, the fear and the odd sense of inevitability that kicks in as you go into labour. The 11 hours between my waters breaking and my son being placed on my chest seemed to fly by, at least in retrospect, at least for me if not for my husband.
I can barely remember the pain, only that measured in ankles it was probably between 2 at the point of starting pushing and 5 at the moment of crowning.  (I walked about on a damaged ankle for nearly 10 years after a tapdancing accident, eventually the ligament gave way and the injury and the subsequent healing process were the most painful things I'd ever experienced - I can estimate how many ankles worth of pain I'm in at any given time!)
My son looks a bit like me, a bit like his dad and a lot like himself (especially as neither of us is bald!). The first 6 weeks were more or less hell but the smiles since then give you a fierce bright feeling in your heart and a rush of love and pride.  Each tiny development is a huge deal: happy exclamations, sticking his tongue out in response to mine, shouting "ah-GOO!" and throwing his arms around his toy dog, rolling from his side to his back on his playmat, trying to hold onto toys.  The cliche is true: each day really does bring something new.
Work seems a million miles away.  Even the lack of sleep is worth it - though you'd have to press my husband pretty hard to get him to admit that too as he really does love sleep- and the sheer delight and pride you feel in knowing that you get to see this little tiny being change from a sleeping and screaming animal to a functioning human with thoughts and ideas and his own independence even at just 12 weeks old... there's nothing else like it. 
Ok, he's woken up and needs feeding. And a nappy change. Waaah!