For an example of what I'm saying, let's discuss the first time that Ted cooled to Connie's constant nagging, obssessive ranting about wanting to get married as well as her off-putting need to change how she behaves to make herself into what she thinks his ideal woman is. Back when they were in University, John would have understood that he had to put some distance between himself and someone who looks like that common stereotype The Clingy Jealous Girl. Add the huffy irritant who put the idea of getting married far too early into things into the mix and next thing you know, his good friend was hectoring him about what a jerk he was for breaking a promise he didn't even make.
My guess is that the reason Ted simply didn't tell John to go piss up a rope is that he hoped that one day, John would realize that Elly is not an honest person and tends to only be happy when everyone else around her is as miserable as she makes herself. This, of course, is why he's an antagonist. Much like Evil Mira, the Evil Career Woman and Evil Becky, he thinks that Elly and the Pattersons need to stop parading around like Lord and Lady Muck.