A chemical reaction to external stimuli indicating another human (or otherwise) as being a fit mate, usually resulting after extended contact can result in more minor forms from less contact. Either grows with time or lessens with time. Sensation akin to heightened dopamine response and a sense of safety and social fitting.
A mind that is stretched by a new experience can never go back to its old dimensions. - Oliver Wendell Holmes Jr.
There's no single definition of love anymore. People overuse the world in so many different situations that it has lost it's meaning. Trying to compile a single definition of love is seemingly pointless, especially if you want an answer that's short and general.