Everybody knows that job interviews are not merely about whether you’re qualified to do a job. If that was the case, companies could just hire people based on a written test and their resume. Job interviews serve the very specific function of determining whether someone would be pleasant to be around for 8 hours a day, 5 days a week. I’m not of the belief that you need to be best friends with your coworkers or anything like that, but if you bring the mood down and are generally unfriendly, that might not be the best thing for an office environment. I know many people want to show up, do their work, and leave, but if you can’t perform the incredibly basic task of being pleasant to your coworkers, you probably won’t last very long. There’s a difference between being shy or private and being openly hostile to anyone trying to have a human interaction with you in the workplace.