Scientific studies tend to view sex and love as a mammalian instinct. Love is an experience that can be divided into three partly-overlapping stage: lust, attraction, and attachment.
Lust is the initial passionate sexual desire that promotes mating. It stimulates the release of hormones such as testosterone and oestrogen. The effect is only last for weeks or months.
Attraction is romantic love stage. People at this stage will constantly release chemical, including dopamine, phenylethylamine (PEA), norepinephrine and serotonin. The effect of this chemicals flowing in bloodstream is similar to stress: increased heart reat, sweaty palms, heavy breathing and etc.
Unlike lust and attraction stage, attachment stagement stage is accounted for long term relationship. Monogamy and trust are related with oxytocin and vasopressin. This is the stage where mature love is.