I think it depends on the perspective: Love can be seen in many ways.
While the guys already described what it is chemically, I'll just say what it is to me. To have a loving relationship, be it friendship, spousal, or even family relationship, you need a lot of balance. Sure, you may love someone, and they might love you back, but is it really love when nothing else fits? They love you, but they would never make a compromise, they would never make a sacrifice, they would never step on their own ego, and admit their faults, can that be called love? I don't know, but I don't.
Sometimes, the love isn't shared, which is terrible, I think a lot of us have been there - you would do anything, you would make all of those sacrifices and things you need to do, but the other person wouldn't - and the lack of reciprocation leads to suffering, and eventually learning a lesson.
I've learned my lessons in time, and my current view on love, is that it's made by millions of little things :). Find someone who has the same spark for you that you do have for them, and start building. It's like building a house, really, each one puts a brick on top of another brick, but if you base your house on a cracked foundation (unfaithfulness, lack of trust, lack of honesty...) the whole thing will collapse. But if the foundation is strong, you keep adding today's laughter and tomorrow's cry, sometimes the bad weather may crash a few bricks, but with a good foundation, you can always put them back.
If something is wrong in the foundations (could be simply finding that you try to build a house with bricks while the other person is adding wood, or concrete) , it will always affect, even destroy the love eventually. (that's why I believe people fighting in early stages of a relationship won't last, something is already wrong).
I believe that true love is ever-growing. :wub: