I have said this before on Mumsnet, but I'll say it again because I haven't seen it said yet.
For me, a lot of having children was honestly about being able to be mindful with my time again, to notice it going by and appreciate it rather than letting it slip away.
When you are a child, summers last forever, a week is eternity to wait, every day is a whole new experience. Every year is quite different, to the point where most of my childhood memories can be pinned down to a specific month of a specific year, even decades later.
Then you enter the working world, you meet someone you stay together with, and life can start to become permeated with a sort of sameness. A whole year goes by and you're still having the same conversations with the same coworkers and you've got to deal with that one trait of your partner's that still drives you vaguely crazy a few times a year but it's all. the. same. Sometimes it feels like being on a treadmill.
You know what's not the same? The way you feel about your sweet, ever-moving cyclone of an 18-month old versus the way you feel about your roly-poly smiling 6 month old. Every year of your children's lives is again differentiated, much the way your own life used to be when you were a child yourself. Baby groups give way to ballet, preschool birthday parties with cake and ice cream change into pizza-scarfing sleepovers. A fascination with dinosaurs yields to a fascination with guitars.
It's too easy in our society to let your prime years become routinized to the point of total, unending sameness. My bachelor uncle told me that he regrets that part -- he has wonderful friends, he has a routine he has followed most every day for 40 years, but now he is having health problems and feels he let those years pass without really even noticing how fast it was happening.
If there's one thing being a parent teaches you, it's how fleeting each moment is, and that they don't come again -- you have to take advantage of each one as they arise.