Pheremones are a sort of biological equivalent of performance enhancing drugs in the olympics. Male sweat puts out loads of pheremones, and male pheremones alter all sorts of things for a female, including their mood, sexual attraction to males in general, even their menstrual cycle. Basically, the more you produce, the better your odds (to a certain point anyway), so it would be selected for.
Without pheremones, the attraction is still there, just not skewed in your favour.
It's all scientific. You don't look at something and then suddenly feel this attraction without something happening in your brain. Electrons or neurons pop around a bit and cross switches and click parts of your brain on. Emotions are thye same. I don't know how it works, but what I do know is that it's all biological.
The exact opposite of love, being rejected, works similarly but very differently. Being rejected sends a distress signal to a part of one's brain and that distress causes physical damage (no joke).
My guess is that love works pretty much the same. Rather than sending a distress signal it sends something more positive and thus, love is nothing more than a signal that your brain recieves to make you feel a little bit better. If your eyes percieve something that your brain finds appealing then the said "signal" will cause your brain to send a feel good shockwave through your body.
something you don't realized you've got until it's already gone :(