If it comes to the point where his sights are only on her without considering the possibility of trying to date anyone else, and she isn’t showing any clear signs of reciprocal interest, society and media has romanticized this situation by calling it unrequited love. Pickup artists, however, have rightfully reframed this condition in a negative light by naming it “one-itis.” It’s when you think she is “the one,” but she either has no idea you think this way, or her level of interest in you fundamentally does not match your level of interest in her.
Guys with one-itis cannot possibly know enough about the women they’re obsessed with to actually be in love with them. By definition, you cannot know if you truly love someone romantically until you see the side of them in which you share romantic intimacy. In a word, it is simply desperation, not love.
Pickup artists had a crass prescription for this disease. If anyone had one-itis, the advice they were given would be to have sex with ten other women. There’s a lot to unpack there. Such an approach can be problematic if it leads you to objectifying women and treating them as nothing more than notches on your bedpost. However, intentionally shifting your focus away from an unhealthy obsession is absolutely necessary. Following this advice also had the benefit of shifting your focus toward a more holistic system of self-improvement (i.e. getting better with women overall), and away from the singular short term goal of dating one specific person. Pursuing such a short term goal would lead to no significant growth or maturity, after all.
When a guy has one-itis, he has an unhealthy attachment to an idealized version of the woman in his mind. This version of her does not exist in reality. It creates unrealistic expectations for what could be. If this guy puts himself out there to actually date human beings from various backgrounds rather than obsessing over a mental image of one person, he will give himself the opportunity to adjust his expectations of human relationships based on how people actually are, flaws and all. It will also temper or even eliminate his desperation, making him more attractive to women in general.
If your goal is to be in a monogamous relationship with one woman, it’s alright to love her and only her, but only after you have established something mutual with her. Any obsession before that is just delusional idealization. In order to avoid one-itis, you need to keep your options open and experience dating multiple people before making a mutual commitment, so that you don’t create any unhealthy attachments.