Well, love is a very natural feeling, and rejection leading to dejection may not indeed be unique.
It depends on how we perceive situations and react to them. This can have an effect on our thoughts and actions and vice versa, leading to a vicious circle.
The solution is simple: We need to cut the vicious circle at any one point either by having positive actions, i.e. attending our routine and friend circle normally, or feeling positive or having positive thoughts. This should persist for some time rather than just a few hours, i.e. probably few days / weeks continuously / consistently / all through the day for any effect to be evidenced.
Sometimes an individual may require help to cut the vicious circle at a point either by a Psychiatrist who can prescribe medications or by a Psychologist who can do some counselling or intervention by both if things are severe.