Let's synthesize what everyone is saying here. There are several good points:
I think what oneitis ultimately comes from is some form of personal insecurity. It can result from feeling inferior to, unworthy of, or incapable of continuing to sustain the attraction of the given female.
First - I am not good enough
To reinforce the previous point, some have mentioned the idea of putting the girl on the pedestal. Maybe this is an issue of semantics, but I don't think that's necessary. Often times that's the natural result of not feeling like you are "good enough", but if you feel that way, you're already putting yourself on an anti-pedestal. Perhaps you feel generally insecure around any woman during a romantic sort of interaction. Are you pedestalling every woman, or have you simply dropped your self image in to a pit?
As Lexington mentioned, another element of oneitis is the idea that there is an irreplaceable quality about the female. Usually the emerges under the context of her having some sort of general archetypal personality compatibility (it feels like "she fits") and may also include her showing some initial interest. Maybe, maybe not. This is a scarcity mindset and since we're all aspiring DJ's and have read The Book of Pook, we all remember what he says about the natural byproduct of scarcity in the context of romanticism - DESPERATION!
Second - I will never find someone like this again
To quote the essence of Pook, there is a lot more riding on the odds of an incidence of low abundance or even a singularity (not a black hole all you physics people) than the odds of an incidence of abundance. I really think this is the pathology of the disease. It begins with the thought that you are not good enough for whatever reason and that there is something inherently special about this female that you HAVE to get her.
As an aside, consider how illogical this is. Most cases of oneitis stem from relatively new encounters in which it would be completely impossible for you to have any rational way to validate whatever assumptions you have made about the female. Most to all of what you know about her are assumptions. And that's a very dangerous minefield to let your mind play in, because it enters from a mindset of inferiority. What will most likely happen is the inferiorly feeling mind will fill all gaps with the most positive pieces of information possibly, regardless of their plausibility and will make conjecture about the female that is also unrealistically and impractically possible. In short,
An inferiorly feeling mind is just as powerful as a supperiorly feeling mind - but - the supperiorly feeling mind will use its power to work FOR you; the inferiorly feeling mind will use its power to work AGAINST you.
Now that's a big problem, because how are you going to get her if you aren't good enough?
As Dust 2 Dust mentioned, oneitis can develop in the infatuation stage of a relationship, and real infatuation may be reciprocated. However, keep in mind what the first stage of oneitis is - a feeling of low worth. If that's going on inside your head, it will manifest itself in your actions. In general, the natural reaction to this seems to be the least productive one - giving increasing amounts of yourself to the female and increasing your overall level of investment in the interaction (or relationship... whatever word works). Some might call this "chasing". What do you do when you find something you think you need but also don't think you can get? You chase it!
Chasing is attraction cyanide
Chasing kills attraction very quickly. Did you know that unconsciousness can present in humans in 10-20 seconds followed quickly by cardiac arrest after ingestion of just ONE rounded teaspoon of potassium cyanide? It doesn't take much.
Some have said that oneitis manifests itself from a lack of options, but I don't believe that. If that was the cause, then the cure would be to find options (ie, spin plates). And this is not always effective. What good is it to spin plates that you view inferior to the plate you have oneitis for? It's also a bad solution because it's trying to address an internal problem with an external solution. Again:
Oneitis is an INTERNAL problem. An EXTERNAL solution will not fix it.
Now back to the issue of spinning plates, this is conjecture on my part, but if you have oneitis, and if it's ultimately stemming from feeling inadequate, and somehow you managed to find another female or 2 that you felt similarly about in comparison to the oneitis female - why wouldn't you also develop oneitis for them? What I think would happen is that your emotions would either disperse unhealthily between the women or one of them would eventually end up winning the title of Queen Oneitis while you summarily forget about the others.
When we realize we have oneitis, we would be best suited to address the internal issue that started the big destructive process in the first place.
Oneitis is an internal problem compounded by a feeling of desperation.
And
External solutions do not fix internal problems.
