Ooh. Toughie.Hmm. Attraction is, IMO, a very fluid thing, which can be heavily influenced by mood, circumstance, and outlook on life, and it occurs when you find something which you want, or admire, such as: someone stable when you are shaky (or stable when you are also stable to keep yourself that way...) or of a like mind to you... As far as physical attraction, that's largely governed by A) what you've decided you like physically in a person, which is B) heavily influenced in some way by society. For instance most people seem to like blondes and I don't generally find them to fall into what I would generally look for in a partner, and I think partially it is because my brain is going: enough people already want these people...why do I have to play into that? (Although I'm really not shallow enough for looks to even be a big criteria but we all make value judgments, particularly when seeking a mate/partner.) So you can end up attracted to the opposite of what society says, but it's still being influenced by that.
And when it all comes down to it attraction (and love, some people would say) are all chemical reactions...
I probably didn't cover all the bases but I did try. 
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rambler
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[This message has been edited by rambler (edited 06-02-2001).]