iven the group focus of many max-level activities in World of Warcraft, social interactions become much more involved and potentially volatile at that time.
The complexity increases with real life relationships start to interact with the gaming ones as well. Some of the worst troubles I've had with groups resulted from someone insisting or expecting that their boyfriend/wife/brother receive a raid spot. Of course, I've also seen numerous cases where this wasn't an issue.
Overall, it tends to become an issue when there is a large gap between one player and the other's goals, or skill; also, when one or the other insists on doing every activity together, often over the wishes of the other person.
An example of the last would be from a run I was in a couple of years ago. Several people were working on their Onyxia attunement, and so I'd set up a raid to go through Upper Black Rock Spire for this purpose. At that point, I knew two players who were very close and did most things together; I'll call them X and Y. Now, X was on, so I mentioned to her that I was going to do a group for this instance, and Y had wanted to come, so I'd like to know if she wanted to come as well. X was unsure, and I informed her that I couldn't reserve her a raid spot without a conclusive answer. She never gave me one, and logged off soon after.
A few minutes after that, Y signed on. I invited him, mentioning that we only had one spot left at this point and that I'd asked X, but she wasn't particularly interested. Y joined the group, and we entered the instance. When we were about halfway through, X logged back on and proceeded to yell at both me and Y. I was accused of trying to steal Y away, among other things. I was stunned by her reaction. Y left to try to placate X, and after that, neither X nor Y wound up being invited to any further group activities.
An insistence on doing things together can be troublesome for any selection of players.This was obviously a rather extreme event. Most trouble tends to come from players bringing in friends or family who don't share the goals (usually killing a specific enemy in a certain fashion), or are not at the skill level needed for what is being attempted. This is rather frustrating to everyone else in the group who is having their goals and expectations thwarted.
Now, there are plenty of cases where the two people share the same goal, or have found activities that allow for differing goals or skill levels. A friend of mine often runs Naxxramas or other raids that he has on farm with his father on an alternate character, while other players make a point of running the heroic daily or doing their daily quests with a specific person. Holiday events, such as the Headless Horseman or Coren Direbrew fights, also offer plenty of opportunity to engage in activities together.
An insistence on doing things together to the point where one player can't engage in any differing goals they have in-game, or trying to do group events with a large goal or skill gap, can be troublesome for any selection of players. Given that a relationship where the participants are close outside the game environment can strengthen the social connection and promote a fear of, say, being pummeled with pillows if they're unhappy with you, such people are harder to deny even if they are ruining your fun.
My wife and i went everywhere together in wow, she was a mage and i was a hunter so we were a double-dps team. She had all the hardcore raider friends and i had the hardcore PVP friends. This was never a problem until they put Hunter and Mage gear as the same loot item. slowly but surely i found myself passing on everything to my wife but at least i was picking up ranged weapons and pole arms. As DPS became more complicated and nerfs more abundant i re-rolled paladin and became a healer. Then suddenly she said it wasn't fun anymore and quit.
but we never had a problem getting groups.
In my old guild we had a boyfriend/girlfriend combo who would insist on doing everything together. They both rolled druids. It was a bummer because one of them was a great healer, but the other was a mediocre tank/DPS. We couldn't bring the great heals without the crappy bear or cat, so we basically 9.5-manned everything.