Mmm... I've heard about the pheromones myself and I've heard that's a reason why you can find people more attractive when you're drunk, because the sensibility of the smell is somewhat less.
To defend the chemicalists (as I've arbitrarily decided to label them ), I've heard of studies that show that people are drawn to opposites in more ways than one and that includes the pheromones people exude. So why would two people appear attracted to each other and then fall apart? Well, we like to use liberal amounts of perfume and cologne in some parts of the world which in some ways cover up or override our natural pheromones.
So two people who've dated a lot always wearing their best face (and odors) seem compatible, but in reality they might have clashing pheromones that are getting covered up. So when they finally move in together and spend time in each other's company without any of the various cover ups, suddenly the attraction is dulled or even reversed.
My philosophy teacher in high school commented that to really get to know someone, you shouldn't be focusing on how they are on a Friday night date, but how they would be on a Saturday morning. That sounds PG-13ish, but that's not how he meant it. His point was that when you go out on a date (or really when you do anything in public) most people put up a facade. And if people fall in love with each other's facades, they may be in for a rude awakening when they meet the real person.
The real person is revealed by the Saturday morning person. How would you be at 9:00am on a Saturday morning if you had made absolutely no plans for the day? How many people would be up, bright-eyed and dressed to the nines at that time? What he was saying is that it's far more important to know what the person really is like underneath (how they would behave on a lazy Saturday morning) than how they present themselves on a Friday night date. So that's a couple of reasons for break-ups and the like.
Anyways... We can give another point of view to what your philosophy teacher said, but that would be to accept that we base our (at least) initial attraction towards someone else on the physical attributes of the other person, and not only on the chemical reactions. So we might feel attracted so certain individual who is very dressed up and covered in perfume on a friday night in a club, but when we see what's underneath that, we're brought back to reality and realize that it was only the appearence what caught our eye, so there was actually no chemical reaction.
So, could there be a chemical reaction between two people who have never met in person? Can we explain the chemical reaction between two individuals that meet over the internet? I don't know, can we? Because I think you can look at someone on TV or a magazine and suddenly feel attracted to her/him (not saying that it's love, but love starts somewhere you know ), is that really chemistry? Because I thought that in order for there to be chemistry there should be at least visual contact between "both" individuals, but I'm not sure.