Well, a broken heart is actually part chemical.  When we fall in love, we trigger the release of various chemicals that give us a kind of high.  We literally become addicted to the other person.  When they leave, our source of a chemical high is gone, and we go through withdrawal.

In conjunction with this, we also usually feel a sharp loss of self esteem and purpose, both of which are triggers for acute depression.  Acute depression can last a long time, particularly if the circumstances that triggered it don't improve.  That's why long after the gut wrenching part is over, we still often feel a vague sense of emptiness.

Of course, this is just a nutshell explanation.  I assume you're not interested in the exact neurological processes, and are ok with this kind of generalization.  If you want more, I can try to dig up some studies.  I'm sure there are lots of free pdfs around the internet since this is such a popular subject.

Anyway, what purpose does all of this serve?  Several, as it turns out.  If you're not aware, the evolutionary description of any plant or animal is essentially a "survival machine" for genes.  That is, we don't reproduce to pass on our genes so much as our genes make us to pass themselves on.  This is necessary for understanding just about any evolutionary adaptation.  Remember that our genes are not particularly interested in what's best for us.  They're interested in what's best for them.  The two are not always the same.  (Of course, when I say they're interested in anything at all, I'm speaking metaphorically.  They have no conscious purpose.)

Human babies have really big brains compared to other primates.  This is good for us once we grow up, but it's bad for the mother and father.  In order to be able to pass the baby's huge head through the birth canal, humans are born far less developed than other primates.  Think of a baby chimp.  Immediately after birth, it can cling to it's mother on its own, and within hougs, is capable of basic motor functions that take human babies months to accomplish.  Because human babies are so helpless, and because they take so long to grow to maturity, they have a high "PI" or parental investment.  Specifically, they have much higher MPI, or male parental investment, than most other primates.  This is just a scientific way of saying what everybody already intuitively knows.  It's a lot harder to raise a kid with one parent than two.

Ok, so what could genes instill in humans such that they would tend to stay with each other for a long time after they make a baby?  Strong emotional attachments, of course!  Our experience of love isn't so intense because we're so smart, at least not directly.   It's exceptionally strong because human parents have a much higher than average need to stay together for a long time to raise their baby together.  So, in short, we get broken hearts partly because our genes really, really want us to stay with our mates for long enough to raise babies.

Consider also that humans are intensely social creatures.  We have very complex extended families, and we form strong bonds with non-kin in ways that are very unusual in the animal kingdom.  Because of our long lifetimes, our reputations last a long time.  Reputation is everything in social animals.  We see this strikingly demonstrated in even relatively unintelligent creatures like vampire bats, who remember which individuals share excess food most willingly, and who refuse to share with those that have been stingy in the past.

In other words, even without the direct motivation of making babies, our genes have a vested interest in us forming long stable relationships.  They inspire this in the same way -- we feel strong negative emotions when we leave close relationships and strong positive emotions when we have ten year celebrations of loyalty and friendship.  This all gets back around to mate selection, of course.  Everything in nature gets back around to reproduction eventually, but in humans, it's  a long and winding path.  People who form long work relationships, who have kept lifelong friends, etc, are demonstrating very desirable qualities to potential mates.  They're likely to stick with it for the long haul if babies do end up being made.

Like I said, all of this is very general, and I'm avoiding anything too technical, but does this basically answer your question?

Yeah that answers it Hamby. I never thought of it that way, thanks man. It sucks but in a way it's kind of comforting to know the reasons behind it.

So take my advice kids, just say NO to love!