Now, let me preface what I am about to say by pointing out that setups are usually a bad idea. In the vast majority of cases, the very fact that your friend needs help finding love is a good indication that your attempt to help is going to fail. This is because the problem is never one of "just not having met the right person yet." It always runs deeper. Maybe she is insecure, or too introverted, or overweight. Maybe she is trying to be masculine, or makes herself unapproachable. Whatever the case, her inability to take charge of her own romantic life isn't merely a matter of bad luck; it is a symptom of a deeper problem, which your attempt to introduce her to someone new isn't doing anything to solve. It's like giving another book to a child with a learning disability, and thinking "this time he'll get it." Exactly. It isn't going to happen.
So with that background, let's take a look at what most people do when they try to set up their friends (because this blog is for women I am going to use the example of a girl, but guys make the same mistake). Once a girl sees a potential match in her social circle, she goes to the girl and guy separately, and tells each of them that she knows "someone that they have to meet." She might tell each person a few things about the other, maybe show them some photos, and she gets them to agree to the setup. Then she arranges some kind of event at which the two people have the opportunity to meet each other. She's "really good at this" because she never lets either person know that the other one knows it's a set up. That way it won't feel forced or awkward. Perfect, right? Wrong.
The problem isn't that the person thinks that the other knows about the setup. The problem is that the person themselves knows they are being set up. And they knew this the very moment the girl told them there was "someone that they had to meet." This does two things: first, it generates expectations and makes both parties feel like they have to perform, which of course results in an encounter more awkward than a new graduate's first job interview. More importantly, however, and what I want to point out here, is that it destroys the single most important thing for someone who struggles with their dating life: autonomy.
Let's look at this from a the guy's perspective...
One of the hallmarks of masculinity is self-control: men want to be in control of their lives, and by extension, they want to be in control of their dating lives. A man will never feel good about himself if he can't initiate and perpetuate his own relationships. (Remember that his inability to do so is the reason his friends want to set him up in the first place.) By setting a guy up, you are essentially stepping in and putting training wheels on his bike - reminding him that he cannot handle himself. Yes, it might be true that he rides poorly (or not at all) without those training wheels; but by taking control of his dating life you are making him feel like a child, and he won't respect himself for any girl he "gets" with your help. He also won't respect any girl that he needs help to meet, because men know instinctively that women are attracted to men who don't need help: men who are in control and confident with their own capabilities. In other words, he knows he can get a better girl if he gets his shit together and deals with his lack of options himself.
You might argue that without some initial help, a guy (or girl) will never date anyone at all. To use the bike analogy, you might argue that, yes, a guy might not respect himself for using training wheels, but without training wheels he will never learn how to ride. But this argument assumes that his problem is balance. In real terms, your insistence on setting him up assumes that his problem is meeting girls. But as I pointed out at the beginning, this is never the case. His problem isn't one of balance, it is the fear of crashing. His problem isn't one of meeting girls, it is the fear of rejection; and setups do nothing to help him overcome that fear.
There is a feminine perspective on this too. When it comes to match-making, the feminine problem is that a girl will not respect a man who needed the help of a friend (i.e. you) to meet and attract her. This isn't only a matter of judging the man's courage, or the social abilities needed to make a connection with her. Women know that if a man is motivated enough, he will dig down and find that courage, and make something happen - or at least he will try. You want a man who desires you enough to push through a crowd to meet you, or takes some kind of initiative. At very least, you want a guy who does more than accept dates that are handed to him because his friends think he has no other options. A girl who finds herself in a relationship that started that way won't respect him, won't respect the relationship, and won't respect herself. So deep down, girls doesn't like being set up any more than guys do. They might like being single even less, but they'll be even less enthusiastic still about being dumped once their boyfriends wake up, and realize that they are only with those girls because they never had the balls to go for what they really wanted.
I am sure many readers know people who have had successful relationships after being set up, some of which might have even lead to marriage. My parents were set up, and they've been married more than 30 years. Maybe you were set up and are still with your boyfriend. And that's fine. But it doesn't mean that those cases are ideal or likely, or that they do anything to strengthen the inner core of the relationship.
Of course, it isn't always the case that the person you are trying to set up has any problems dating or attracting the opposite sex. Maybe they have plenty of options, and you just happen to know someone who would be a great match for them. But in these cases, you won't need to do anything other than introduce the two people anyway; they will be perfectly fine on their own if there is an attraction, so the strategy is the same: introduce, step back, and let it happen. Nothing more.
1. You Are Responsible For Your Own Romantic Happiness