Heinlein defines true love as when someone else happiness matters more to you than your own.
This, while romantic, clearly, has issues. Namely, it implies a certain sacrificial quality bound to create resentment at times, even if both involved are constantly vaulting to put one before the other. It also seems to go against a certain survivial instinct, and against a certain instinct to find happiness in love, unless the sacrifices necessary in this definition also imply a feedback happiness from your lovers happiness that would outweigh the initial sacrifice.
A friend of mine (thanks GB!) extended this definition to incorporate certain buddhist tenants: true love is when someone elses comfort and security mean more to you than your own ego. I found this a particularly wonderful notion.
Recently a friend of mine was describing some of the intricacies of a particularly unconventional relationship between two people we knew. This relationship, to me, is one of the few examples I have seen in which an open relationship, or polyamory, seems to work. He noted that the concern the husband had for his wife's happiness included wanting her to feel attractive, and this extended not only ecnouraging sex with other men or women if she wanted that to feel that way, but to feeling sympathy and comforting her when she experienced turbulance with others. This latter part, which seemed ripe for discomfort on his part, was also profound to me on some level. Touching. I couldn't really put my finger on it at the time, but when my friend threw that definition forth it made perfect sense. The reason that relationship worked wasn't the quirky and unusual ways in which they chose to define their relationship, but an ongoing attempt to put eachothers comfort and security before their own ego.
I've seen this in another equally unconventiional polyamorous relationship, but in a completely different manner. The couple in question: both extraoridnary, attractive, exciting, enthusiastic, and social were like playmates in love and they shared the details of their other daliances rather openly. One member of this union told me, once, that her partner had requested she not see someone she was interested in, because she seemed to have too big of a crush on him. This, to me, struck me as something that could cause a lot of tension and could be seen as a little too selfish, there he was telling her she could have sex with anyone, but not the one she most wanted. She responded it didn't bother her at all. She was happy to do that for him, it never even entered the realm of question. At the time, this also struck me as moving: they had defined unusual parameters in which to live and love and express their persionalities, but their freedoms ended where the actions of their egos might threaten the others comfort and security. And they knew that. They even knew they could request such concern in the other without question.
This is not, by the way, my little essay on why polyamory is best. I don't believe that any more than I believe the reverse. I've seen just as many egocentric and hurtful open relationships as I have monogamous ones. For ever frustrated, trapped and confused stereotypical couple I know, I have seen "open" couples who consider such flexibility carte blanche to take whatever they want, whenever they want it, and label any attempts to rein in these impulses as breach of the contract, unfair limits on their freedom.
I guess my point is, it's easy to be selfish in any arrangement. And it's easy to like someone and want someone and to call that love. But maybe true love is really measured by how much you are willing to manage yourself and your own ego's desires. How much joy you can derive from reigning in your own ego to see another person truly find comfort and security, not just happiness and passion.