Job hopping is a career killer.
It makes job searching harder and can cost you a lot of opportunities.
Don't just take it from me. Here are a few choice excerpts from articles and blogs posted on the subject by experienced managers, CEOs and others who have built up some authority on the subject.
But first, a survey by Bullhorn Reach among 1,500 recruiters and hiring managers found that:
According to 39 percent of recruiters, the single biggest obstacle for an unemployed candidate in regaining employment is having a history of "hopping jobs," or leaving a company before one year of tenure.
Before we delve into it, a quick note: the quotes presented here do not necessarily match my own opinion on the subject. Some are particularly dismissive of job-hoppers and you should keep in mind that these are blog articles where strong opinions abound. Combined, I believe that these articles illustrate that a history of job-hopping is a significant hindrance in a job search.
Because I can't and won't reproduce entire blog posts here, I'm only copying the sections relevant to this question. I encourage you to read the full articles; without context these people's comments can come across as harsh or even condescending.
Noted workplace advice columnist and manager Alison Green explains why job hopping is a problem in her US News article Are You a Job Hopper?:
Savvy interviewers believe that the best predictor of how someone will behave in the future is how they've behaved in the past — their track record. So if someone has a pattern of leaving jobs relatively quickly, an interviewer will assume there's a good chance they won't stay long in a new position either. Since employers are generally hoping that anyone they hire will stay for at least a few years, a resume that shows little history of this is a red flag.
Venture Capitalist Mark Suster describes job hopping and his opinion of it in a blog post Never Hire Job Hoppers. Never. They Make Terrible Employees:
It’s kind of like that famous saying about art, “you know it when you see it.” If you’re 30 and have had 6 jobs since college you’re 98% likely to be a job hopper. You’re probably disloyal. You don’t have staying power. You’re in it more for yourself than your company. OR … you make bad decisions about which companies you join. Yes, if you were a startup CEO I would probably cut you some slack. Yes, 2% of you have legitimate reasons for having 5 jobs. But in a competitive job market you’re less likely to get the chance to tell me your sob story.
Oleg V. Volkov rightly pointed out in the comments that people naturally are "in it for themselves", we work for a salary after all. I feel like the author phrased this poorly and it would be more accurate to say that job-hoppers give the impression that they're only in it for themselves, and don't care about the problems they cause by leaving jobs very early.
I should also point out that the full article has a much more detailed explanation of his reasoning and some excellent advice on salvaging a job-hopper's resume.
Nick Corcodilos of Ask The Headhunter® is particularly expressive in his article, Job hopping: Career crack for losers
Any job hopper who’s fool enough to be one of 1,000 resumes on some manager’s desk deserves to be dumped into the trash can. Gimme a break — your work history shows you bounce around like a ping pong ball and you expect a manager to overlook it until she gets to meet you in person to see what a wonderful, unique individual you are and that your job hopping was due to extenuating circumstances that you can explain, given the opportunity?
Just stick a fork in your butt — trust me, you’re done. You not only job hopped, you’re advertising it to the world by applying for jobs with a resume. Do you really expect a manager is gonna “understand” when she doesn’t even know you? You are revealing that, on top of being a job hopper, your judgment sucks.
Pretty harsh, huh? As before, the full link has advice on how to repair your work history.
Repairing your work history
How you go about repairing a history of job hopping deserves its own topic, though there is already a related question on this site. I'd also recommend reading this article by Alison Green. As mentioned, the articles linked above all provide advice on recovering from a bad work history.