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 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.

Jungians view the process of falling in love as one of projecting the anima or animus onto the other person, with all the potential for misunderstanding that this can involve.

Two chemical reactions associated with falling in love are increases in oxytocin and vasopressin; and Elisabeth Young-Bruehl has suggested that "when we fall in love we are falling into a stream of naturally occurring amphetamines running through the emotional centres of our very own brains". With regard to sociobiology, it is stressed that mate selection cannot be left to the head alone and must require complex neurochemical support.

Critics of such Neo-Darwinism point out that over-simplistic physical arguments obscure the way sexual passion often leads not to secure attachment but to attachments thwarted, as well as the sheer frightening difficulties of all falling in love.

Biologist Jeremy Griffith suggests that people fall in love in order to abandon themselves to the dream of an ideal state (being one free of the human condition).^{[citation needed]}