Consider the inner observer that watches your antics, usually when you're upset.  If you and I are each aware of an inner observer, it's likely that everybody has this property and capacity.  (Of course, we need a certain level of awareness, and we must choose to look for it.)  That every person has this inner observer points to its universal nature.

What must it be?  Let's look at something analogous: Consciousness.  Consciousness is non-material, incorporeal.  It is utterly without boundaries.  What if you, consciousness and 'spirit' - the immanent reality that religions speak of - are the same?  This view does not depend on myth or supernatural beliefs, but rather on deep inner exploration of who you are.

Now ponder something else.  Who were you before you learned to differentiate yourself, before you constructed your identity?  (Don't expect a quick answer; for now, just know that by changing your consciousness, you can know the answer.)

This inner observer is something far more enduring than our normal self.  And it's ever-present.  It's our True Self.  And as you explore the essence of who you are, the nature of love - and 'heart' - assume greater importance.  Ultimately, they're foundational to living our lives at the level we know we're 'called' to do.  Something deep within us tells us overpoweringly that we have no choice.

From this perspective, how can we knowingly be unloving to anybody?  If they are 'us', how can we harm anybody, whether through word, act or omission?  When we recognise that the higher self of the other is identical to our higher self, then we treat them with the same kindness we feel towards ourselves.  This is the foundation of the Golden Rule - do to others as you would have them do to you.  Without engaging in metaphysics, the True Self described here is also the foundation of Christianity's teaching that 'God is love'.

Of course, at times we'll fail to live up to this ideal.  One reason is that we're mostly unaware of this higher self, so awareness practice is needed.  Another reason is that we have shadow (blind spots).  Yet another is our egos.  These areas will be discussed a little later on this site.