Birthdays are always days of change, even if we don't expect it. We get another year older, wiser, hopefully kinder and better, then we were just the day before.
Birthdays can change everything about you, and about the world. Take mine, for instance.
I had no idea when my friends came to celebrate just how things would end. I had no idea that a day begun with cake and presents, would end with a journey to stop an evil I had never heard of, a legend I didn't know, from destroying everyone and everything I ever cared about. I didn't know that on my birthday, I would have to embrace not only my destiny, but all my potential destinies, all of our potential destinies. Now me and my friends are exploring lost ruins, soaring through the sky on forgotten ships, and facing down monsters that I can barely imagine, and can just barely survive, all to prevent the very literal end of the world.
Suddenly, getting older doesn't look so bad.