Re: how exactly do chemicals produce the exact
The chemical and neurological mechanisms are not completely understood yet, though we do know some of the neurotransmiters and hormones involved with forming emotional bonds and some of the brain regions which specialize in our relationships with other individuals.
But I'm not sure the exact chemicals involved are really your question, if you're just trying to understand the true nature of love. Who you fall in love with and when will depend on a lot of things; yes, what we have evolved to value in a mate will be part of it. Some researchers have evidence that the smell of people with diverse immune function genes compared to our own is attractive to us. But it's important to understand that as humans, we have a long history of assessing our environment, assessing our needs, and adjusting what we're looking for in a mate accordingly. And compatible personality is actually an important factor; you want to make sure that you and your mate will get along long enough to successfully raise your offspring to adulthood (and maybe even beyond) together. That's especially true in today's western societies because a woman doesn't necessarily need a man or vice versa in order to successfully raise her offspring. If anything is going to keep them together it will be their enjoyment of each other's company, and even if two parents are not required, they're still better than one.
I hope that helps a bit. I'll wait and see your response since I'm still not sure exactly what you're looking for.