From Wikipedia (Limerence - Wikipedia, the free encyclopedia)...
"Limerence refers to an involuntary cognitive and emotional state of intense romantic desire for another person. The term was coined by psychologist Dorothy Tennov to describe the ultimate, near-obsessional form of romantic love.
The concept is an attempt at a scientific study into the nature of romantic love. Limerence can often be what is meant when one expresses having intense feelings of attachment, preoccupations with the love object, and (as new research on brain chemistry shows) a similar mind-state to obsessive compulsive disorder. 
Limerence is characterized by intrusive thinking and pronounced sensitivity to external events that reflect the disposition of the limerent object towards the individual. It can be experienced as intense joy or as extreme despair, depending on whether the feelings are reciprocated."
"Tennov differentiates between limerence and other emotions by asserting that love involves concern for the other person's welfare and feeling. While limerence does not require it, concern may certainly be incorporated.
Affection and fondness exist only as a disposition towards another person, irrespective of whether those feelings are reciprocated, whereas limerence demands return. Physical contact with the object is neither essential nor sufficient to an individual experiencing limerence, unlike one experiencing sexual attraction....
Limerence can be longer-lived than transient forms of romantic feelings such as infatuation and puppy love, enduring for months, years and even a lifetime in the absence of knowledge about reciprocity...
Limerence involves  intrusive thinking about the limerent object; acute longing for reciprocation; if unrequited, transient relief may be found by vividly imagining reciprocation; and fear of rejection and unsettling shyness in the limerent object's presence.
As well, the feelings of limerence are intensified through adversity, obstacles, or distance, and the person may have acute sensitivity to any act, thought, or condition that can be interpreted favorably, and an extraordinary ability to devise, fabricate, or invent "reasonable" explanations for why neutral actions are a sign of hidden passion in the limerent object.
A person experiencing limerence has a general intensity of feeling that leaves other concerns in the background, and in their thoughts, they tend to remarkably emphasize what is admirable in the limerent object and to avoid any negative or problematic attributes.
During the height of limerence, thoughts of the limerent object (or person) are both persistent, involuntary and intrusive. Limerence is first and foremost a condition of cognitive obsession. All events, associations, stimuli, and experiences return thoughts to the limerent object with unnerving consistency.
The constant thoughts about the limerent object define all other experiences. If a certain thought has no previous connection with the limerent object, immediately one is made. Limerent fantasy is unsatisfactory unless rooted in reality, because the fantasizer may want the fantasy to seem realistic and somewhat possible.
Fantasies that are concerned with farfetched ideas are usually dropped by the fantasizer. Sometimes it is retrospective; actual events are replayed from memory with great vividness. This form predominates when what is viewed as evidence of possible reciprocation can be re-experienced (a kind of selective or revisionist history).
Otherwise, the long fantasy is anticipatory; it begins in the everyday world and climaxes at the attainment of the limerent goal. A limerent fantasy can also involve an unusual, often tragic, event.
The long fantasies form bridges between the limerent's ordinary life and that intensely desired ecstatic moment. The duration and complexity of a fantasy depend on the availability of time and freedom from distractions. The bliss of the imagined moment of consummation is greater when events imagined to precede it are possible.
In fact they often represent grave departures from the probable. Not always is it entirely pleasant, and when rejection seems likely the thoughts focus on despair, sometimes to the point of suicide. The pleasantness or unpleasantness of the state seems almost unrelated to the intensity of the reaction..."
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