Beautifully put by fusion and Cornish: love is THE THING.
And not only the love they give you, also seeing them show another person love: a sibling, an ailing grandparent, a friend in difficulties- and knowing that this is what they learnt from us, this is something they have taken from the home we gave them.
And that moment, particularly when things are tough, when one of them makes a comment and everybody laughs and you think to yourself "this is something that wouldn't mean anything to anybody else, this is special to us because we are a family".
As for the actual parenting, what I found was that there was no way of judging beforehand which bits I would struggle with and which bits had sounded terribly scary but actually I was fine with them.
I was terribly worried that I wouldn't be able to cope with the physical intrusion of the baby, it taking up a room which I had used as my own private space. When it came to it I found it didn't feel like that, because the baby was still almost part of me: it would be like complaining that my leg or my arm took up too much space.
Sleep deprivation also wasn't too bad: not everybody has a colicky baby that screams through the night.
Breastfeeding otoh was much harder than anticipated: I thought after all my preparation it would be a walk in the park, but there were health issues and it all became quite stressful.
Otoh I remained surprisingly calm throughout my second high-risk pregnancy, prem induction and emergency caesarean.
Mostly I didn't mind being constantly needed: I rather enjoyed the fact that there was this small dependent person that I could soothe with my very presence. I liked singing to them, I liked reading to them, I liked making them laugh. I enjoyed the nappy changes because they were such a social thing.
I found I was ok with toddler tantrums, am also fine with teen backchat, but there was a period of whining round the age of 3 which I found quite hard: I used to sing a lot to drown out the noise. Also a period of pre-pubertal gloom and doom around the age of 10/11 which rather got on my nerves: it was like walking round with your own personal rain cloud. Teenagers otoh are the salt of the earth as far as I am concerned.