Considering All is One... it would seem that all evidence to the contrary is merely illusion (i.e. the idea that there can even be an "other" to have a relationship with). And yet... every single relationship - between the various parts of what make up that Oneness - is sacred, Divine at essence, based in love and eternal. The trouble lies in our ability to perceive this essential truth... we tend to distort it as Daisy mentioned, through various aspects of our human make-up. So our perception of relationship, at this level, is illusion. The relationships themselves are not, nor is what we gain through exploring them.
Like so much else here, really.
"The most important decision you have to make is whether you live in a hostile or friendly universe."
~ Albert Einstein
The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. ~Marcel Proust
The evolution of humanity is an evolution of the heart. The path is through the heart.