  Praise their partner. No, seriously, it works. I used to joke that I helped break up two marriages.
It was an amusing correlation that I drew. I start talking to a person having marriage problems, but is sticking it out. Then later they reach the point that they break up with their spouse. Now, I didn't actually blame myself for it, as in both cases the spouse (both men) was not being a good husband, even to the point of being emotionally abusive/manipulative.
More so it was as I said, an amusing connection. However, it lurked in the back of my mind that maybe I did affect things somehow, and something I read recently solidified that feeling. The book I was reading is called urlLink How to Win Friends & Influence People . Doesn't quite sound like a book to get ideas on breaking up couples, now does it? Bear with me. Part III, Chapter 1 is titled: You can't win an argument. The reason for this, is that when you argue with someone, you are putting them on the defensive. Even if you succeed in the logic, and defeat them, you still likely haven't converted them to your way of thinking. Why? People are emotional beings. As such: A person convinced against their will, is of the same opinion still.
So, how do you convince people? The book recommends agreeing with them. Once they are no longer feeling the need to defend their positions, they are open to your ideas. Ok, so how does this relate to breaking up relationships? Simple. Most people that want to break two people up, will go to their friend and try to point out all the bad things about their partner. This instinctively causes the friend to become defensive and try to rationalize and play down the things their partner has done.
Now, I've been accused of being a nice guy, and as such I try to look for the good in people, and try to look for ways to help people work out their problems, where I can at least. Looking back, what I now believe I was doing was giving my friends a chance to, for themselves, see the bad traits in their partner.
They've always known they were there, they likely even told other people about those traits. But usually those other people will likely agree with their friend's assessment, causing the friend to second guess, and make excuses. With my habit of not attacking my friend's partner, and even trying to point out the good, my friend would finally have a chance for their brain to process the bad traits without being defensive. It's an interesting theory at least. Now, don't read what I've written the wrong way. First, I don't blame myself for breaking them up, as in both cases I believe the relationships were unhealthy.
Second, this technique will not work on a fundamentally healthy relationship, due to the fact that the negative traits must actually be present before they can be acknowledged. Side note: One of said friends came up with the second half of the title to this post. 
