Love between an young man and an young woman has many things
which are not obviously known to conscious mind.
The Oxford English dictionary describes love as: an intense
 feeling of deep fondness or affection for a person or thing 
and to fall in love as: to develop a great love for. 
This may well be a basic description of what love feels like,
 but why do we love, what is passion, and why is intense 
desire between two people sometimes called 'chemistry'?Heart Box
There are, in fact, three distinct stages of love; each with 
their own characteristic emotional profile and scientific explanation.
First is lust. Lust is driven by our sex hormones testosterone 
and oestrogen. These hormones are what get us 'out on the pull'.
 After lust comes attraction. This is the love-struck phase; 
the time when we lose our appetite, can't sleep, and can't
 concentrate. This is what we know as falling in love. 
http://www.thenakedscientists.com/HTML/articles/article/clairemcloughlincolumn1.htm/
Imagine an invisible, undetectable force that's powerful
enough to override your sense of reason yet draws you 
to someone with an almost animal passion. These aren't 
Cupid's mythological arrows, but real shots of human 
pheromones.
http://health.discovery.com/centers/sex/aphrodisiacs/phermones.html
Welcome to The Scent of Eros:
Mysteries of Odor in Human Sexuality -
'Scientists in Philadelphia have established for the first
time that the human body produces pheromones, special
aromatic chemical compounds discharged by one individual
that affect the sexual physiology of another'....
http://www.athenainstitute.com/discovery.html
Do pheromones work in human sexual attraction?
They may be odorless and colorless and their function may be
mysterious, but human pheromones at last have the zest of
scientific truth. Researchers at the University of Chicago
have demonstrated that compounds swabbed from the
'underarms' of young women at different times of the month
can alter the length of other women's menstrual cycles,
compressing or expanding the cycles in predictable
fashion....
http://www.ishipress.com/humanodo.htm
http://www.cnn.com/HEALTH/9803/11/pheromones/index.html
 What is the force that lights the fuse between two 
complete strangers? What current pulses through their veins, 
engorges their hearts, occupies their minds and numbs their 
texting fingers? Ah, if only
I knew. As magic formulas go, sexual chemistry must surely 
be the most valuable. It's intoxicating effects are more 
pleasurable and more powerful than any drug and when it hits, 
it hits hard. There are the rushes of ecstacy, the gut wrenching 
anticipation, fluttering, dizziness, and the diminished 
concentration that turns day-dreaming into a full-time job.
 And then there are the side effects. A positively Colombian 
annihilation of appetite and its confidence- boosting by-product,
 weight loss. And it is free. And it is legal. It is bloody amazing.
But no one knows what "it" is. Scientists have managed to
 map our genetic blueprint. They understand the subtleties
 of hormones and the complexities of the emotional brain. 
But the thing that makes two people click remains a mystery.
 Theories abound as to what, or why. Those looking for a
 magic bullet (or Cupid's arrow) tend to favour the notion
 of pheromones - scents secreted by the sweat glands in the 
'armpits' and pubic hair. And the relatively recent discovery 
of the vomeronasal organ, a small chemo-sensory structure in
 the human nose, lends the concept some weight. 
http://findarticles.com/p/articles/mi_qn4159/is_20030629/ai_n12741733
Science of Love - Cupid's Chemistry:
There are, in fact, three distinct stages of love; each with 
their own characteristic emotional profile and scientific explanation.
First is lust. Lust is driven by our sex hormones testosterone
 and oestrogen. These hormones are what get us 'out on the pull'.
 After lust comes attraction. This is the love-struck phase; 
the time when we lose our appetite, can't sleep, and can't 
concentrate. This is what we know as falling in love. 
http://www.thenakedscientists.com/HTML/articles/article/clairemcloughlincolumn1.htm/