It's not something that can be explained. You have to figure out what it means on your own, even if you find that it doesn't mean anything at all.
Untrue! Love can be explained quite simply through biochemistry.
That is, it derives from the hypothalamus, in the brain (lower region, I think) which interacts quite heavily with the nervous system (which explains some symptoms of "love", such as heavy breathing and sweating). The hypothalamus communicates with the pituitary gland of your brain, which releases a slew of chemicals (hormones) into your body which in turn create a kind of a high (such as the release of serotonin, the body's natural "feel good" chemical). In other words, it's a perfectly natural
drug.
This follows through with the fact that when your "heart is broken", as they say, this release of chemicals is halted and you go through a kind of withdrawal, among other physiological symptoms that probably derive from the psychology of the situation (which is, of course, related directly to the psychology of the person!).
As to what causes this, we don't really know. Most researchers believe it derives singularly from the sex drive, but others think it has higher meaning than being purely instinctual. I'm inclined to believe in the first option (since the pituitary gland doesn't release the chemicals itself, but instead interacts with the sex organs, and
they cause the release of the chemicals), but there's lots of other details that are quite beyond me as a scientist.
I will say one thing, though (and I point this out every chance I get), but
chocolate does many of the same things to your biochemistry as "love". ;)