There is a theory of human behaviour known as Hygiene Theory, due to Hertzberg (1962, The Motivation to Work, Wiley, New York). It was developed to consider the subject of motivation to work, and especially applied to remuneration. In summary it says this: someone will view the prospect of additional benefit (say increased pay) with desire; he will be pleased for a time when he receives it; but after a while he gets used to having it, and will begin to want more; if it is reduced it will be regarded as a loss, which is painful. After a time this loss becomes accepted and the pain is reduced.
Research on this theory has been done mainly in the field of work and pay, but clearly will apply to other things. In the case of having or acquiring a boyfriend/girlfriend it appears that the Hygiene factor operates very quickly. That is to say, if one sees a desirable man/girl in the distance then the prospect of getting together is viewed with desire. But a few minutes later when the interactions have built up to a high level (or even before it has built up to a high a high level), the Hygiene factor has switched over into the state of assuming that the relationship will take place. If anything now prevents the relationship progressing it will be regarded as a loss, and probably a very painful one.
We regard these two attitudes of mind as very important in the courting situation. When it is Only to gain there is desire, spirit of adventure - or the "thrill of the chase". Unfortunately, this state may only last a short time on sighting a new mate - unless some factors are present to prolong it. It is soon replaced by an Only to lose attitude of mind - which is a very different psychological state from Only to gain. The prospect of a loss is feared, and people under fear do not perform well - they are tense. The appearance of a loss is painful, causing people not to think or behave rationally.
We can see this syndrome playing a part in several previous syndromes. In particular, if factors are present that prevent the rapid switch over from Only to gain to Only to lose people will be happier and perform and interact with each other more effectively.
Consequently in the Sue/Carol Syndrome, if the girl knows early on that the man has a girlfriend elsewhere - before the switch has occurred - then she remains in the Only to gain state. She remains happy, the interactions build up, and she may end up getting him nevertheless. But if she learns of the man's girlfriend after the switch into Only to lose, she treats this as the prospect of a painful loss, is tense and unhappy, and the interaction between them falls.
Likewise in the Di Syndrome when she already had a man, there is no prospect at that moment of getting off with the new man - that's clearly in the future - so the situation is entirely Only to gain. When she meets him and she doesn't have a man the situation has switched to Only to lose. There are also other factors at play here to be dealt with in the second study.
Hygiene Theory also indicates that after a time the mind accepts the state of loss, and begins to view the prospect of any advance as a gain once more. This may be part at least of the explanation of why in several of the syndromes a girl shows a delay between avoiding and advancing again. But the delay in switching back to Only to gain is often a long one.
It is naturally better to keep ones own and the girl's Hygiene factor in the Only to gain state as long as possible, and to reduce the time spent in Only to lose before a date is offered. - so everyone knows where they are.
This underlines the need to offer a date when high interaction is reached, and avoid the risks of the various adverse syndromes, especial Isobel.
A way of prolonging Only to gain is the need to achieve high interaction before a date is offered. Thus on seeing a new girl merely have an objective of seeing if such high interaction occurs by saying something to her. If it doesn't, then leave to another occasion and try again - stay in Only to gain.