Oneitis (also known as limerence and, depending upon other factors, unrequited love) describes a state of consistent romantic obsession with a person who is not in a relationship with you. A oneitis is often unrequited, can last for months, years, or even decades, and can be psychologically devastating for the person suffering it, leaving them only in the realm of fantasy.
Oneitis has many causes. Anyone can suffer oneitis, even those who are very socially active and have no problems in the dating realm, and much literature has been written on the topic. The most intense oneitii, however, tend to occur when a person has few romantic opportunities, few indicators of interest from the opposite sex, and/or is socially isolated.
Oneitis is psychologically unhealthy. Time spent obsessing over a oneitis can interfere with almost everything, and severely hamper personal progress. The effect of unrequited love can lead to mental health issues. A degraded mental state, combined with obsession, can also cause a person to not pay attention to legitimate interest from others, making oneitis a vicious circle.
Solutions to oneitis are complicated by the lack of understanding from those who do not suffer it. In this sense, it parallels the general lack of understanding of love-shyness and incel in general. One solution to oneitis is psychological repression, which is aided by filling one's life with other activities, especially those involving new people. Oneitis tends to worsen with social isolation, so it must actively be worked against. The only real solution to oneitis is to actually fall in love with a person.