In chemistry, a reactant is an initial component, such as an atom, molecule, ion, chemical species, or compound, etc., at the start of a chemical reaction.  In a generalized sense, a reactant is a substance that is consumed in the course of a chemical reaction.  The "products" of a reaction are considered as the end components. This is diagrammed below:
Reactants → Products
In human chemical reactions, the description of what is a “reactant” in a reaction requires some thought and depends on the energy characterization of the process. In a simple combination reaction, such as when an unacquainted pair, A and B, collide in time, begin to date, and fall in love over a set number of months or years:
A + B → AB
the reactants are generally considered as the human molecules in an unattached state prior to first contact.  A typical example, would be a two people (reactants) falling in love (products) at first sight.  According to two studies, 28% of Americans have fallen in love at first sight.  Similarly, 65% of people agree with the statement: "falling in love was not really a choice; it just struck me." 
1. Daintith, John. (2004). Oxford Dictionary of Chemistry. New York: Oxford University Press.
2. Reactant – IUPAC Gold Book, 1996.
3. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview). Morrisville, NC: LuLu.
4. Naumann, Earl. (2001). Love at First Sight - the Stories and Science Behind Instant Attraction. Naperville, Illinois: Casablanca Press.
5. Fisher, Helen. (2004). Why We Love - the Nature and Chemistry of Romantic Love, (pgs. 23, 42). New York: Henry Holt & Co.