Because the world is changing and we are changing, all the time, forever and ever. And sometimes, by holding on too hard, we lose. Loss can be ugly, beautiful, scary or all of the above but I have found that it is, more than anything, necessary.
So when people leave I have lerned the secret: let them. Because most of the time. they have to. Let them walk away and go places. Let them have adventures in the wild without you. Let them travel the world and explore life beyond a horizon that you exist in. And know, deep down, that heroes are not qualified by their capacity to stay but by decision to return.