Good advice tends to fall into two categories: either so specific that it is of no use in any situation other than the one it was originally intended for, or so general that it is hard to figure out how it might apply to any specific situation.
This advice falls into the latter category.
And here it is: Show True Love. Or, put in other words, Be A True Friend. If you like another and they don't like you: be a true friend. Give them space. Be kind, but don't assume that that is going to lead to any particular results. If they like you and you don't like them: be a true friend. Be honest--I've had talks a few times with girls who thought that they were being "nice" when they didn't break up with a boy, because they didn't want to hurt the boy's feelings. My question was always: when do you think this is going to be less painful for him? After you've broken off an engagement? After you've been married for a year? But also: be kind-- no character attacks. No true friend would rip on another's self-worth on top of breaking their heart.
And still another way of putting it (thank you to Immanuel Kant for this phraseology) is to say that you should treat others as ends unto themselves. This sounds really weird until you think about the expression "a means to an end." Using things as a means to an end is fine: you take a class as the means to get to the end (or, ultimate goal) of getting a degree. If you use a person as a means to an end-- if, for instance, you suppose that one person a) can and b) should meet all (or even most) of your social and emotional needs-- then you are treating that person badly. People are not roads to be walked on to get to your ultimate goal. They are people, and as such they deserve to be sought out and associated with for themselves-- or, left alone, because they prefer to be and you are willing to respect them.
For me, I cannot claim (yet) that following this advice has produced a boyfriend, or even likely prospect of one (though I can cite numerous stories in which it comes shining through-- like the ending sequence of Pride and Prejudice). On the other hand, I can tell you with certainty that when I have managed to keep it, this advice has made my life happier, more peaceful, and overall more worth living. After all, even when the one I currently have my eye on wanders off to more likely prospects, I am left with something I get to keep: an improved character.