It's partly the maths. Our son is two weeks old today. Two weeks! A lifetime. For me, aged 34 and a half and a day (blame Adrian Mole, I am slightly obsessed by age fractions) those two weeks are just 1/897th of my life. What a small proportion of my life he has been here for so far. Had I had him when I was, say, 20, these two weeks would have been 1/520th of my life. No wonder they say they grow up so fast.
It reminds me, and forgive me while I take some time to bawl my eyes out, of Fiddler on the Roof's Sunrise Sunset: "I don't remember growing older/When did they?/Sunrise, sunset/Sunrise, sunset." One sunrise and sunset, that's just 1/12601 of my experience.
In Linda Grant's brilliant Remind me who I am, again she is reminded by an expert in care of the elderly that her mother was someone before she was her mother, just as with dementia she was someone other than her mother again. Each incarnation different, but as valid as the other. My children will not think of the thousandths of my life before they existed (and nor should they particularly) but I will, not with regret, because to have that life before being a parent was great, but with a little sadness for the missing fractions.