Other People's Lives

Have you ever noticed how in other people's lives things seem to happen so much more quickly, like a blink of an eye?  Take your eye off a long-distance friend, and before you know it, she's engaged, married, or expecting a baby!  Speaking of babies, other people's children grow up so quickly: you see them a few months later, and it's like they're totally different little people (and they don't remember you at all).  Why is that?