Overview
Falling in love is the development of strong feelings of attachment and love, usually towards another person.
The term is metaphorical, emphasizing that the process, like the physical act of falling, is sudden, uncontrollable and leaves the lover in a vulnerable state, similar to "fall ill" or "fall into a trap".
It may also reflect the importance of the lower brain centers in the process, which can lead the rational, accounting brain to conclude (in John Cleese's words) that "this falling in love routine is very bizarre.... It borders on the occult".
Factors
Mental
"Factors known to contribute strongly to falling in love include proximity, similarity, reciprocity, and physical attractiveness", while at the same time, the process involves a re-activation of old childhood patterns of attachment. Deep-set psychological parallels between two people may also underpin their pairing-bonding, which can thus border on mere narcissistic identification.
From Wikipedia (CC BY-SA 4.0).