There's an old Times-article here
that goes through the topic in some depth.
I'll also present parts of my own view of this. Remember, all of these are the basic biologic systems - there's some cultural conditioning on top. This is just where the *impulses* come from.
First, let's try to get the basics in place: The attraction system - and all other systems - are formed by a single imperative: What makes genes get reproduced to further generations. The overall reproductive success measurement is how many great great grandchildren do you get (grandchildren is actually included in the direct feedback loop, so your influence on them will go on to great grandchildren through only one level.) This seems to be why humans live so long.
The attraction/love system was formed a long time ago; if we're talking the last common full ancestor, that's about 200,000 years ago. However, for the *basic* functioning of the system, we're talking of the limbic system, which goes back before we separated from the reptiles!
Now, to further reproduction, there's somewhat different imperatives for men and women. In both cases, it's about getting your genes paired with other genes that make them survive, keeping the context in which your offspring live safe, and maximizing the use of the available resources.
For females, the restricted resource is how many kids they can have and provide for. The system will attempt to get a paired male of as high value as possible (for providing well for the kids), yet not so much higher than the female that he will run away after getting a better female. In periods of fertility, the system will tempt her towards males with higher genetic quality, as pure gene donators - as long as she don't get caught, so she still keeps the provider male. Her major jealousy factor is towards the male as provider, not sex per se - as the male can service an almost unlimited number of females.
For males, the restricted resource is the females willing to accept his seed, and his time for supporting children. There's several strategies he can use, depending on his "genetic quality". If he's got high genetic quality, he can rely on just spreading it around, having sex with many females who either take care of the kids alone, or that were cheating on another male and have that male help them. Males that follow this pattern is called "alphas". If the male hasn't got the "genetic quality" to do the spreading, he can do second best: Pick a full time mate that will accept him, stay loyal enough to her that she'll keep accepting him, keep her from other males, and impregnate other females when he can. Males that do this are called "betas". The primary jealousy objects for males are if the female has sex with other males or if he risk losing the female as mother for more kids. For betas, there may be the additional point of risking not being able to care for his children (ie, losing invested time.)
The male will thus go for fertility, and wish to "trade up" when that fertility is spent, as long as that increase his overall number of progeny.
A lot of the above is indicated through heuristics. Both genetic quality and assumed provider quality is found by a combination of looks, subconscious smell, confidence, and social proof (how *others* seem to perceive the person - why just judge yourself, when you can add in the combined judgement from a lot of others?) That's a large part of how social skills come into the picture.
There's also a bunch of aspects looking for something that is "like ourselves", for a variety of reasons (including how species creation happen.) There's one aspect (I know of) where we look for somebody that's specifically different: Immune system components.