chemicals in the brain. you need to learn more about guys through real life experience than the internet - it's a BAD idea.
BUT i will help u a bit anyway...
The way it works is that the male brain has evolved to attempt to spread as much of thier own genes as possible. think about it - a mutation that creates the desire to reproduce itself is going to survive better than one that says to never reproduce at all, isn't it?
now the reason women have evolved to be more conservative with sex is simple. it's to do with childbirth.
if a woman had the genetic mutation that created the urge for sex without discerning the personality of her partner and building a bond, she would have f*cked him and lost her partner, facing pregnacy and childbirth alone in the wild. think about how long it was before animals developed civilisation that supported single mothers.
Hope i could be some help but in future, trust me, learn these things from real life or books or close friends or figure it out urself, bcus the internet is really sketchy