Honestly, I think it is one of three, maybe four things..
1. Jealousy. They could be jealous of the fact that you are single, and free, and wouldn't be tied down like they are.
2. Misery loves company. Let's say, for example, John's situation isn't so great with their partner, and he has a friend, Ted, who is just a living reminder of how good his life could have been if he were still single. It wouldn't be long before John starts to think that if his friend Ted, were in a relationship like he is, then Ted, would be in the same situation, and from John's perspective, wouldn't be flaunting his [Ted's] great single life in his face all the time. I'll use my personal experience.
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I am a 28 year old woman who is engaged to the same man I've been with for over 9 years. I do not have children and am therefore have the freedom to do as I please because of it. However, every time I talk to my cousins down south, (whom are 1 and 2 years younger than I am) they almost always bring up 1 of 2 topics, even if I don't, and that wasn't what we were talking about at all. When I call them, it is usually to say hi, and to see how they are doing. They talk about their kids, I talk about my job, and school. They ask me if I had been to any clubs recently, and I tell them no, and remind them that it really isn't my cup of tea. Their response always segues into the first of the two topics..
The first is, how much they'd like to come up and visit me because they want to enjoy the nightlife here, and hang out with me and how when they can go out and I can babysit for them since I am not into the club/bar scene. My cousins have 9 kids between them, and not by the same father.. and so, finding a babysitter for them would be next to impossible especially if some of the fathers are incarcerated, can't be bothered or if they are busy with their other kids. It would be easy if they were by the same fathers because then the fathers could watch the kids, and they could come up for a visit, but since that is not the case, it is not as simple. Anyway, my cousins are always the ones who tell me how lucky I am that I don't have kids, and how I am free to do as I wish, and how I should come down and visit them since it is easier for me to do so.
I agree with them in that it is easier for me to travel to see them, and if it weren't for the fact that they are always mentioning how I could babysit for them if they visited here, and I have a sneaking suspicion that they may try to pull something like that when I am down there, (I heard that they did this to their mom and our aunt, and that is sort of the reason THEY refuse to babysit for them unless it is an emergency.) I would. Otherwise, I don't agree with them.. it may seem easier, but I too, have responsibilities. I both work and go to school full time, and I have other responsibilities as well. And just because I don't have kids and have more freedom, doesn't mean that my duties take a backseat.
The second, is how my fiancé and I should have kids, so that our kids can play together someday. While that is a nice idea and all, my fiancé and I are no where near ready to have kids. We have many responsibilities, and are trying to get our lives and finances in order, so that, not only is our schooling taken care of, but to save up money to buy a car, pay for our wedding and honeymoon, and to also buy a house. We don't have anything against kids, it's just that we refuse to bring kids into the world that we cannot take care of. We do enjoy our lives as it is now, however, we also know that at this point in our lives are not conducive to raising kids. Now normally, I wouldn't jump to the conclusion that they are jealous of the act that I don't have kids, but considering the fact that they have a tendency to talk behind peoples backs, and word travels fast, and I have heard what they have had to say about me, I can.
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3. Eliminating a threat. For some god-forsaken reason, people who are married, or in relationships, see single people as a threat.. and are out to "take what's theirs." It is a reason why so many people believe that if they are married, that they can and should only socialize with people who are also married. This is why some women insist, that their husband drop the friends he's known since BEFORE he met her. Some will have a valid reason, but some won't. I don't know if they've taken leave of their senses the moment they became a "we" or if their relationship is so devoid of chemistry, that they look to create it, by creating drama, where there really is none. I don't know, and I don't know why that is, but, for whatever reason, it just is. These people may believe that if YOU the single person have someone to "distract" you, that you won't set your sights on what "belongs to them." And therefore, to them, you will no longer be enemy number 1.
Now, I realize that the first 3 points may come across as cynical and/or negative, and so, I'll end this on a positive note.
4. Sharing the wealth. In other words, they were miserable before they found their partner, and are now over the moon with joy, and are disgustingly happy now that they have, and believe that because you are single, that you aren't happy, and are just as miserable as they were and they want to spread as much happiness around-- even if you didn't ask them to spread it. It is similar to the Jehovah's Witnesses, Born Again Christians, and Mormons doing door-to-door conversions. They are happy because they have finally been "saved" and when they were miserable before, and believe that because you aren't "saved" that you are miserable as well, and so, they taken it upon themselves to "share the wealth" so to speak.
Personally, I live and let live. I don't treat my friends any differently now, than I did before. I don't try to play cupid. I instead, leave people to their own devices. If my friends have some time, and they want to get together, then, if I have the time, I will. If my fiancé wants to hang with his friends, I'm ok with that. The only thing that would be a concern, is whether or not we have something planned at the time they call.. and the same thing goes for me and my friends.. otherwise, it is not a concern. Neither of us sees our friends as a threat, and we don't treat the single ones any differently than the ones who are taken.
And this is coming from a soon-to-be married woman.