It's when you hurt when they hurt.
When they cry, and you feel that stab in your soul.
It's not some teenage infatuation, or some pop-music ideal of love. It's sharing the sorrow that they have, and both wanting to take that pain away, and yet understanding that they have to undergo that pain on their own, yet still suffering with them.
Every description of love seems to involve the glassy-eyed, Romeo and Juliet, "Let's die for each other!" kind of love. I've seen every few novelists actually get into the painful side of love; the part where you hurt in your soul because your loved one hurts. I suppose it's because so much writing today is done in a politically correct, pussy-whipped sparkly-vampire novel, and that sucks, because it takes away so much of the actual experience.
But then, if the writers haven't experienced it, how can they write about it?
Meh. I've drank to much.