We Came As Romans- An Ever-Growing Wonder

This song is saying that we can be whatever we want to be, by saying "I am the man in the mirror" the singer means that he is himself and he can be whatever he wants to be. When they say the thing about empty glasses filling each other he means if you put someone down, it's like making them an empty glass and they cant make others feel better when they already feel bad about themselves. Therefore creating a chain of hate. They want people to give compliments to fill another person's glass. That way they can "Fill another person's glass" by complimenting them. Therefore making the world a better place. This is just what I think though.