Personification is the assignment of the qualities of living things to inanimate objects. This literary device is used in the above line from Romeo and Juliet.
First, Morning is described as being grey-eyed. Morning has no eyes not to talk of being grey-eyed. This attribute of living things was attributed to morning to describe its newness.
Secondly, morning was described as smiling. Now, again only living things are known to smile. This smile represents the brightness that comes with morning.
Lastly, the night was described as frowning. This describes the darkness that comes with the night.
Thus, the author used personification in this line.
