For time waits for no one, and The Reaper claims us all eventually. And so we trick it, leaving behind the closest thing there is to ourselves. This is life, this is our way to refuse death.
Why, you ask? Because we don't want to die. We are alive, and we want to live. This is our defiance in the face of entropy.
Of course, this doesn't even need rationalization. Do what you were born to do, your instincts guide you.