1. You were qualified, but someone else was more qualified. In this job market, employers generally get flooded with well-qualified candidates, which means that an awful lot of qualified people are getting rejected. You might have been great; someone they'd have been thrilled to hire if Candidate B didn't happen to be better for the role. It's important to remember that getting a job isn't just about being a great candidate—it's about being the best candidate, and it's impossible to know from the outside whether that will be you or not.
2. You weren't as qualified as you thought you were. Job seekers often mis-assess their own match with a job, either because they don't understand what the employer is really looking for or because they overestimate their own skills and experience.
3. You turned off the hiring manager in the interview. You might be qualified on paper, but that won't matter if you blow the interview. And that could take one of dozens of forms: Maybe you seemed rude or arrogant, or you didn't answer questions clearly, or you rarely made eye contact, or you seemed unprepared for the conversation.
4. You weren't a culture fit. You might have all the qualifications an employer is looking for, but still not get hired because your working style would clash with the people with whom you'd be working. Remember, it's not just a question of whether you have the skills to do the job; it's also a question of fit for this particular position, with this particular boss, in this particular culture, and in this particular company.
5. You weren't able to articulate why you'd excel at the job. If you aren't able to make a strong, compelling case for why you'd be great at the job, the interviewer isn't likely to put one together on her own. Interviewing successfully usually means laying out past experiences and skills that equip you to tackle the job, as well as a track record of doing well at work that uses those skills.
6. You were annoying. Constantly checking in for updates, pushing for an offer before the employer is ready, or calling with detailed questions about benefits before you even have an offer are all good ways to make a hiring manager think that you'll be a pain to work with.
7. You didn't seem enthusiastic about the job. Employers want to see that you're excited about the job and engaged in discussing it. No one wants to hire someone who doesn't seem especially interested in the opportunity.
8. You only seemed interested in what the job could do for you, not what you could offer the company. If all your questions focused on pay and benefits rather than the details or the work, and you seemed more interested in how quickly you'd be able to move up than in what you'd be doing every day in the role you were interviewing for, chances are good that the company chose to move forward with a candidate whose primary focus was doing the work well.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.