Insult of witnesses
Young people may simply respond negatively on the quality of his companions in the presence of friends. This can be compared with another woman (of course, not in favor of his girlfriend), or simply a negative review of some of its quality or appearance.
In this case, simply joked in response to the surrounding not witnessed your argument. And left alone with a young man, find the cause of this behavior. For example, it may be his little revenge for what you told him something unpleasant couple of days ago.
All the fault of alcohol
Cavalier can not calculate the forces and just drink in the company of friends girl. In this case, you must take him away and try to comfort her. If the gentleman had already cause harm to their behavior, apologize to host a party.
The next day, explain the beloved, that he is very upset with you and you are not comfortable in front of friends who just called and asked his health. This conversation should make him think about his behavior. But if such a situation will recur, then the problem a little deeper, and there must be intervention specialist.
Delay
It happens that the girl with the young man make an appointment, where she wants to present the gentleman to your friends, but he did not appear. That the situation was not too humiliating joke, for example, that a loved one was abducted by aliens, and try to relax in the company, without thinking of the unpleasant incident.
When you meet your new beau, ask why he did not show up. Maybe he just forgot about the upcoming meeting. That such a situation does not recur, it is necessary to remind the young man about your plans and call up a couple of hours before the event.
Scandal
The young man, who came to visit with friends for dating his girlfriend, can begin to dispute that, of course, will be unpleasant and friends, and to the girl. Try to change the topic of conversation to neutral.
When you return home, explain to the gentleman, what is the problem. But it must be done gently, so that he did not think that you have accepted someone else's side in the dispute.