Finding out the subject did not reciprocate.
Watch the film "Blue Valentine" and you'll see what the other responders mean. Falling in love is a wonderful, lifetime journey, but a relationship that is left to wither and die is one of the most crushing experiences a person can have.
Not falling in love...
Knowing that the person that you fell in love with is one of your siblings or long lost siblings that you cannot marry at all.
Falling out of love.
Finding out that the person you fell in live with killed herself (that really did happen to me)
Being in love is the only socially acceptable form of insanity...
falling out of love