"Estimation Friends" (doubtful)...

I don't know if the words I'm going to say are true or false, but my experience and the years in that I've meeting very good people, and in all relationships, there's almost always an "estimation friend". I decided to call them like that, but I also could call them "doubtful friends" or other names, but the thing I really want is to refer is friends that cause disagreements in a relationship... That exists in my relationship, and I think that this happens in many other relationships... But what friends are they? I think they're that friends very close to only one member of the couple, where the other tends to see suspiciously. But, curiously that person is normally close to the couple, and although apparently all seems to be okay, in the reality the things are not so like that. The jealousy or the possible threat that we imagine in our head, never pass and always remains, and with the minimal oversight, we're back to charge against the "estimation friend". It will be like that?... To think... :)