Just like it didn't really happen if you didn't tell someone about it, for me it didn't really happen if I didn't write it down. Because what's in my mind is ephemeral; it could be tipped out at any moment, to be replaced by children's shoe sizes or where I last saw your orange dinosaur.
Sometimes it's because I need to remember all the things I mustn't forget. Sometimes it's because I need to find out what's inside my head and it won't come tumbling down, like Rapunzel's hair, until my fingers fly along the keys. And sometimes it's because I get overwhelmed by the fact that every few years we lose a generation's worth of stories, of knowledge, of information.
I want to tell my parents' stories, but they're not mine to tell. I'd like them to tell their own stories, but they don't feel the same compulsion I do to send the words out there. They might even feel a desire to keep things quiet, untold; even if that means they're lost forever. Some of their stories are probably lost already. Some of mine are too.
Writing it down makes it real. It keeps it alive. It takes it out of our heads - those amazing, wondrous, unfathomable places that are as fragile as mere skin and bone and grey matter - and turns it into marks on a page that can be deciphered today, tomorrow, perhaps long after we've gone and our bones have nourished a thousand rose bushes.