Apart from their inherent wonderful-ness, ably detailed by posters above (the baby smell! The incredible beauty of them! The amazing cleverness and fun of everything new all the time!), there is what they make of you.
You may already be one of those people with a strong sense of self, and a sense of their being a point to your life. I wasn't particularly, despite having a lot going for me and a lot of options. But now I am, all the way down to my bones, A Mum. and I love that, knowing who I am, what I'm for, what I want more than anything else to do.
I love the capacity it gives me - to love, to cope, to be patient, to be kind. When you stand there dripping from head to toe in someone else's sick and don't give it a moment's consideration at the time, because all you want is to comfort them - when you reflect on it later and realise that just comes naturally to you now, you can feel that you are becoming a better human being because of them. I was told my mother had died unexpectedly the other week, just as my partner was bathing our little girl; and I was devastated and cried and cried. for exactly half an hour; then I dried my tears and went upstairs and read her her story and put her to bed, as I have every night of her life, because she needs me to. And she has made it possible for me to cope with this horrible thing that would have turned me inside out with maudlin angst when I was childless, because not only does her need for me keep me going in spite of the pain, she actually replenishes my soul and makes me happy when nothing else could. It's a perfect feedback loop so far - she needs me to be her everything, and so I find I can, because she does.
People talk about 'getting your life back' a lot after you have kids. But this life I have now is better than any I had before, so frankly you can keep it! 