Because I need to be reminded of life. Birds, by their very nature, are life itself: graceful and delicate.
Because I need to be reminded of happiness. The music of hundreds of birds is joyful, not tainted by fear or pain.
Because I like to take care of them. What better joy is there than to provide a basic need for something so uncomplaining? It fulfills some intrinsic craving I have to make everything ok for everyone.