Obviously some people are going to instantly feel a strong romantic or sexual connection with a person, and then have that connection blossom into a loving relationship. Of course, some people are going to be hit by lightning multiple times in their life. Guess which one if more common?
So in practice, while it might be a wonderful concept, it's not going to happen.
It happened to me and my gf. We caught each other's eye across a crowded room. That was eleven years ago and we're still going strong (living together for the last nine years).
I could see right away that she was very pretty, very smart, and not one to play stupid games (failure to spot the latter is the cause of much suffering imo). And there was something else - some quality that I could never quite put into words, but which I found immensely attractive. It was also pretty clear that she found me attractive too.
Of course, we didn't fall in love right away - we had to spend some time getting to know one another first. And it wasn't always plain sailing either. But, although I wouldn't call it love at first sight, it certainly wasn't just lust either.
The reason stuff like this happens, I believe, can be found in the subtleties of non-verbal communication.
Working with autistic kids has given me a better understanding of the power of the thousands of tiny signals we give off. These kids have trouble understanding such signals, which results in them having great difficulty in many social situations. And, what's more, when asked about it, they often express their total bafflement at how non-autistic people manage to communicate so easily with one another. It's almost as if people have some strange psychic powers, which allow them to have entire conversations without actually saying a word.
So, while I am very sceptical of the idea that one can truly fall in love at first sight, I think it's important to remember that a single look can convey a whole lot of information (most of it unconscious - but then attraction and love are predominantly unconscious).
Bear in mind just how much of the brain's processing power is dedicated to the skills required for finding and securing allies and sexual partners. The ability to size someone up in an instant, comparing the subtle cues they give off against an unconsious 'wish-list' of desirable traits, is pretty fundamental to human relationships in general, and romantic relationships in particular.