(4) is common among people who move internationally more than once or twice. Making friends as an adult is already difficult, and knowing that you will move on after a couple of years makes it even more difficult. You have a reason to socialize with your coworkers, so they will become the center of your new social life. If you moved to a popular expat destination, you may be able to find other expats who are similarly disconnected from normal life. Beyond that, making friends requires crossing cultural barriers, which takes a lot of effort and extraversion.