Recently, someone revealed that he is "in love" with someone who is in a relationship. I commented that I don't believe in unrequited love. I think two people can fall in love and never be together, but I don't think it can be love when the feelings aren't mutual. If there's no reciprocation, the person who thinks he's in love is fixated on the idea of this other person being with him. It's infatuation, not love. It's putting the other person on a pedestal, thinking she's the perfect person for you, when you haven't been with her to know her quirks and flaws. You don't know what it's like to be intimate with her, and you won't get the chance because she's in a relationship (at least if it's a happy one, and not failing already).
This person said that he liked certain qualities of her personality and the fact that she makes him happy with ease. That's what friends do. The fact that the friend happens to be the gender you prefer doesn't make it a romantic relationship. That idea is only coming from one side. She's with someone else; she isn't looking for intimacy with you.
If he's waiting for her relationship to fail, he does not respect her. Who wishes the pain of a failed relationship and a breakup on someone, just so he has a chance of being with her? That's selfish, and love isn't selfish. It's unkind, and love is not unkind. It's an "If I can't have her, nobody can" mentality.
There are plenty of single people out there. Finding someone compatible is difficult, to be sure, but if you're looking at people who already believe they've found someone compatible, you are looking in the wrong places.