When we can truly be happy for another person's happiness, the sense of feeling separate dissolves. From this place of boundary-less-ness, we begin to experientially understand a quiet joy that is invisible and much bigger than we could ever imagine.
Why the Buddha compared the physical world to an empty mass of form. Understanding why the five aggregates are empty and void of essence. Based on a sutta in the Samyntta Mikaya: "Form is like a lump of foam, and consiousness like a magic show."