I am a bit of a cynic on this front. I believe we have the biological capacity to 'fall in love' - strong infatuation characterised by heightened levels of hormones like phenylethylamine and oxytocin, producing symptoms similar to OCD - on a reasonably regular basis as one of the ways the species perpetuates itself. What this experience 'means' in someone's life is mediated by their personal, social and cultural history and setting.
When it is experienced in relation to someone of the 'right' age, social setting, availability, etc, this can be the start of a lasting relationship. The quality of any resultant relationship depends on a lot more than the 'being in love' - personal, social, family compatibility, wanting the same things, shared history, attitudes to commitment. When in inauspicious circumstances (teenager and pop star? best mate's girlfriend?) it is in our culture labelled 'just a crush', though the feelings may be just as intense.
When 'being in love' is believed to be the most important thing for a relationship, some people manage to sustain those feelings long-term. Most don't, and I think it is a tragic indictment of the central place romantic myth in our society that when these initial feelings start to fade away (or worse, start to be felt towards someone else), perfectly good relationships get wrecked because of the central importance we attach to the 'being in love' experience.
I've been with dp since we were teenagers and in that time have felt 'in love' feelings with several other people - normally at times when dp and I were rocky, or there were other things going on in my life that I wasn't happy with. It's at times been incredibly painful but each time I've been glad in the long run I didn't do anything about it. So far, for me, hanging on to the love, respect, history, family and wanting the same things that dp and I share has been more than worth giving up the ephemeral, irrational obsessiveness of regular fallings-in-love. But then, like I said, I am a bit of a cynic
Sorry, turned into a bit of an essay there!!