Angels look towards the light so that the shadows fall behind them. Angels live indefinitely, not eternally. After some time period like 200 or 500 years they will take human incarnation or they will be dissolved.

Some angels do not want to come down to a manifested form, with a gross human body.

After a few centuries they want to be freed from their form. They get tired of playing their roles and also they do not want to play their role eternally. So they take human incarnation on some other planet to live. They have worked hard, so they want to enjoy retirement. Angels never say goodbye for the last time.