The reason for this is that people who do that tend to not realize that they don't actually know what the problem with the other person actually is. The problem that Ted currently has is that he's about to realize that Connie was primarily interested in him for his money. While Lynn clearly entertained the fantasy of telling a penitent Doug that he missed his chance and has to suffer for having left her for a big-busted brunette, the message she's actually sending by having her fall for a much richer banker is Ted's love is as nothing as to Greg's money. John is too busy listening to the very biased and very ignorant Elly's take on events and viewing his alleged friend through the prism of his own issues to see him for what he is and his situation for what it is. While he does later go on to be dead on about Ted's idiotic refusal to admit that no, Irene shouldn't be expected to keep house while he plays around because all his protestations that the flings were meaningless does is make her question her own meaning to him.
In the here and now, however, John has done nothing but antagonize his drinking buddy by telling him that what he thinks is wrong with him is actually what's wrong with him. Connie was quite willing to be with Ted until his letting Mommy do his thinking for him and fear of leaving Mommy to her own devices like a 'bad' son made the situation untenable so blathering away witlessly about what he can't have adds insult to injury.