copy pasta:
for most people its a simple chemical reaction. one that eventually dies out, which is why divorce is so prevalent.
people that manage to stay together even after this chemical reaction has died out either hate each other and are together for cultural/social obligations, or they develop a different level of love that is very different from fleeting, ordinary, romantic love.
love is too imprecise a word to accurately define universally though. too many different emotions get lumped under 'love'. although they may seem similar, the love you feel for your significant other isnt the same emotion you feel towards your mother, your dog, your favorite instrument, your precious collectors item, or whatever it is that someone can claim to 'love'. by the same token, hate can be closer to love than most people realize. it is part of the reason why abusive couples are able to stay together for long periods of time.
since you are most likely asking about romantic love, its most important to remember theres no such thing as eternal romantic love. its entirely chemical, and doomed to dissipate. people get caught up in myths and other such stories about famous lovers, and they forget humans dont live forever. is it possible for two people to love each other romantically their entire lifetime? yes, if they live very short lives. couples who continue to love each other towards the limits of human age do not experience the same love they had when they were young. their body chemistry changes. no form of love is a static state, and the longer romantic love lasts, the more likely it is to change into a sort of familial love, which is entirely different. you could debate the terms, but in summary, love between two people at age 20 is not at all the same at 120, plain and simple fact.
contrary to what the previous poster has written, infatuation itself is the classical idea of romantic love. in fact its probably romantic love in its purest form. love that endures after infatuation is no longer pure romantic love.
the notion that you cant 'fall out of love' is nonsense and is the assertion of one who has had little romantic experience, or a long string of repeated failed ones.
ultimately, you will have to experience different forms of love for yourself to understand what the different emotions mean to you. try not to give too much weight to semantics, and dont be naive.